Check this out. I'm very sorry to not have tried that before!!
This is the collector voltage waveform at turn-off for a $1 MJE13009 bipolar transistor after having charged the load inductor to 12A during the on-time. Voltage scale is 50V/div (from 100:1 HV probe). Vce for Ic=12A is just 0.8V for a 5:1 forced gain.
Now try to get that 50ns rise time with the bulky bank of 500V MOSFETs required to get 0.8/12=0.066 ohms rds-on without special gate buffers!!
And this is the test circuit that made the miracle possible (in breadboard!):
BTW: Turn on is also damn fast and free of storage/saturation losses because the energy stored in the 1uF capacitor during turn-off is used back to boost the base drive during the first microsecond of the conduction period. Actually, what the circuit does is to transfer back and forth all the base charge between the transistor and the capaciror
This is the collector voltage waveform at turn-off for a $1 MJE13009 bipolar transistor after having charged the load inductor to 12A during the on-time. Voltage scale is 50V/div (from 100:1 HV probe). Vce for Ic=12A is just 0.8V for a 5:1 forced gain.
Now try to get that 50ns rise time with the bulky bank of 500V MOSFETs required to get 0.8/12=0.066 ohms rds-on without special gate buffers!!
An externally hosted image should be here but it was not working when we last tested it.
And this is the test circuit that made the miracle possible (in breadboard!):
An externally hosted image should be here but it was not working when we last tested it.
BTW: Turn on is also damn fast and free of storage/saturation losses because the energy stored in the 1uF capacitor during turn-off is used back to boost the base drive during the first microsecond of the conduction period. Actually, what the circuit does is to transfer back and forth all the base charge between the transistor and the capaciror
Hi Eva,
This principle was afaik described in the 80´s / early 90´s in the German "elektronik" magazine. I must have a copy anywhere.
Nowadays you may use one device as seen at:
http://www.st.com/stonline/prodpres/discrete/powebipo/esbt.htm
There may be other manufacturers too.
Regards
Onra
This principle was afaik described in the 80´s / early 90´s in the German "elektronik" magazine. I must have a copy anywhere.
Nowadays you may use one device as seen at:
http://www.st.com/stonline/prodpres/discrete/powebipo/esbt.htm
There may be other manufacturers too.
Regards
Onra
I know ST "ESBT" modules, but they are both expensive and very hard to find while their specs are not quite suitable for what I want. Also they are more aimed at operating from 380V, 415V and 450V AC rectified industrial mains voltages (650V DC) instead of 230V conventional mains (350V DC). On the other hand, the two devices that I employed are industry standard, very easy to find and very cheap, yet they yield excellent performance.
These are the datasheets for the four ESBT models that ST appears to manufacture:
HYBRID EMITTER SWITCHED BIPOLAR TRANSISTOR - ESBT" 1500 V - 3 A - 0.55 Ohm
HYBRID EMITTER SWITCHED BIPOLAR TRANSISTOR - ESBT" 1700 V - 3 A - 0.55 Ohm
HYBRID EMITTER SWITCHED BIPOLAR TRANSISTOR ESBT 1500 V 8 A 0.075 Ohm
HYBRID EMITTER SWITCHED BIPOLAR TRANSISTOR ESBT" 1000 V - 50 A - 0.026 Ohm POWER MODULE
Note how all except the bulky latter one are not suitable for a 230V AC SMPS due to the low current rating. In contrast, my device combination is rated at 700V 12A 0.012ohms, is truly useable at Ic=12A without reliability issues and costs $2. I'm not aware of any other manufacturer producing ESBT, as everybody seems to be very focused either in IGBT or MOSFET stuff now.
And yes, I'm also aware that emitter switching was introduced in 1980s, and indeed I think that it has not received all the attention that it deserved.
These are the datasheets for the four ESBT models that ST appears to manufacture:
HYBRID EMITTER SWITCHED BIPOLAR TRANSISTOR - ESBT" 1500 V - 3 A - 0.55 Ohm
HYBRID EMITTER SWITCHED BIPOLAR TRANSISTOR - ESBT" 1700 V - 3 A - 0.55 Ohm
HYBRID EMITTER SWITCHED BIPOLAR TRANSISTOR ESBT 1500 V 8 A 0.075 Ohm
HYBRID EMITTER SWITCHED BIPOLAR TRANSISTOR ESBT" 1000 V - 50 A - 0.026 Ohm POWER MODULE
Note how all except the bulky latter one are not suitable for a 230V AC SMPS due to the low current rating. In contrast, my device combination is rated at 700V 12A 0.012ohms, is truly useable at Ic=12A without reliability issues and costs $2. I'm not aware of any other manufacturer producing ESBT, as everybody seems to be very focused either in IGBT or MOSFET stuff now.
And yes, I'm also aware that emitter switching was introduced in 1980s, and indeed I think that it has not received all the attention that it deserved.
Hi,
Pardon, but your headline sounded like "Heureka"
My intention was to point out, that this circuit is well known.
It is basically a cascode of a FET and a grounded Base bipolar Transistor.
Major advantages are faster switching than a grounded Emitter and higher Breakdown Voltage.
Worse is the need of a separate power supply for Base current.
Onra
Pardon, but your headline sounded like "Heureka"
My intention was to point out, that this circuit is well known.
It is basically a cascode of a FET and a grounded Base bipolar Transistor.
Major advantages are faster switching than a grounded Emitter and higher Breakdown Voltage.
Worse is the need of a separate power supply for Base current.
Onra
AndrewT:
The need for an additional power supply depends on how your drive the base. In ST application notes you may probably see constant base current drive through an additional supply. In contrast, in my circuit the base current is obtained through a current transformer, thus no additional supplies are required, except for the 15K biasing resistor that is required to put some charge into the 1uF capacitor to allow the circuit to turn-on the first time.
Onra:
I said "I'm very sorry to not have tried that before!!". Indeed, I could have tried that a lot of years ago, but I was stupidly regarding it as impractical Also, don't get me wrong, my headline should sound like "how really old stuff can outperform brand new stuff".
The need for an additional power supply depends on how your drive the base. In ST application notes you may probably see constant base current drive through an additional supply. In contrast, in my circuit the base current is obtained through a current transformer, thus no additional supplies are required, except for the 15K biasing resistor that is required to put some charge into the 1uF capacitor to allow the circuit to turn-on the first time.
Onra:
I said "I'm very sorry to not have tried that before!!". Indeed, I could have tried that a lot of years ago, but I was stupidly regarding it as impractical
Also, don't get me wrong, my headline should sound like "how really old stuff can outperform brand new stuff".Hi,
Eva wrote, she built her circuit on a breadboard.
Therefore i think she did a simple and quick work to demonstrate the function.
The separate power supply may be part of the main PSU, but a saturated BJT needs an IC/IB ratio of 10, some of down to 5 (like Eva did). Working with Collector currents of 20A and more requires a base current of at least 2 to 4A during switch on. Switch off is done by a required negative Base current of about 6 Amperes and more, depending on switching speed.
I looked up the App-Notes at ST and found the AN9464, which contains some scope screen shots (currents and voltages)
http://www.st.com/stonline/products/literature/an/9464.pdf
Hope, this will help.
@Eva: Did you perform a research "Back to the roots"?
In my opinion the amount of necessary devices may have prevented this circuit from common use.
Regards
Onra
Eva wrote, she built her circuit on a breadboard.
Therefore i think she did a simple and quick work to demonstrate the function.
The separate power supply may be part of the main PSU, but a saturated BJT needs an IC/IB ratio of 10, some of down to 5 (like Eva did). Working with Collector currents of 20A and more requires a base current of at least 2 to 4A during switch on. Switch off is done by a required negative Base current of about 6 Amperes and more, depending on switching speed.
I looked up the App-Notes at ST and found the AN9464, which contains some scope screen shots (currents and voltages)
http://www.st.com/stonline/products/literature/an/9464.pdf
Hope, this will help.
@Eva: Did you perform a research "Back to the roots"?
In my opinion the amount of necessary devices may have prevented this circuit from common use.
Regards
Onra
The problem with emitter switching is that the base of the top transistor briefly sees the full collector current during turn off. As can be expected, some transistors don't like this treatment, and will fail after a while. Too bad, as you can get pretty spectacular results otherwise.
ST has just come out with a series of transistors expressly designed for emitter switching, meant to replace >1000V MOSFETs, which are huge and expensive. It'd be interesting to find out what they did to tailor the transistor for ths kind of operation (if anything), and whether it works as well as advertised. If I remember, I'll also ask a colleague at work who has more experience with emitter switching to go into some more detail about the conditions where he experienced transistor failures. I've personally used emitter switching with a Philips BUP23C, but never saw any failures that could be attributed to the base giving out. However, the design never went into production - maybe I was lucky...
ST has just come out with a series of transistors expressly designed for emitter switching, meant to replace >1000V MOSFETs, which are huge and expensive. It'd be interesting to find out what they did to tailor the transistor for ths kind of operation (if anything), and whether it works as well as advertised. If I remember, I'll also ask a colleague at work who has more experience with emitter switching to go into some more detail about the conditions where he experienced transistor failures. I've personally used emitter switching with a Philips BUP23C, but never saw any failures that could be attributed to the base giving out. However, the design never went into production - maybe I was lucky...
poobah:
I proposed that in another thread, but I haven't still tried it. Due to the N-MOSFET-feeding-a-P-bipolar nature of IGBTs, I have doubts about how much a slow "standard-speed" IGBT would take to saturate when there is another device shunting it and preventing it from receiving enough drive current, and how much would it take to desaturate back when the C-E voltage is not allowed to rise.
Onra:
Yes, I built it in a breadboard because I wanted to investigate the turn-off behaviour, as my other bipolar base drive system has some drawbacks that I'm desperately trying to overcome. That thread covers most of my previous work with bipolars: http://www.diyaudio.com/forums/showthread.php?s=&threadid=60969
wrenchone:
My circuit is not exactly like the ones shown in the application notes. See the 560pF feedback capacitor across the MOSFET and try to figure out what it does. See also the 1uF base capacitor, that in hard switching conditions could easily destroy the B-E junction, but mine is not emitter-hard-switching...
Actually, the first thing that I found out is that hard switching over the emitter was not going to produce the performance that I wanted. Paradoxically, with bipolar transistors the fastest turn-off behaviour with no current-tail is only obtained when the rate at which charge is extracted from the base is intentionally limited, and the optimum value seems to be somewhat smaller than Ic. In the shown working conditions I got no current tail at all as the emitter does not seem to get pulled up during the collector voltage rise period. I have to verify that for lower currents and lower forced gains, though.
I proposed that in another thread, but I haven't still tried it. Due to the N-MOSFET-feeding-a-P-bipolar nature of IGBTs, I have doubts about how much a slow "standard-speed" IGBT would take to saturate when there is another device shunting it and preventing it from receiving enough drive current, and how much would it take to desaturate back when the C-E voltage is not allowed to rise.
Onra:
Yes, I built it in a breadboard because I wanted to investigate the turn-off behaviour, as my other bipolar base drive system has some drawbacks that I'm desperately trying to overcome. That thread covers most of my previous work with bipolars: http://www.diyaudio.com/forums/showthread.php?s=&threadid=60969
wrenchone:
My circuit is not exactly like the ones shown in the application notes. See the 560pF feedback capacitor across the MOSFET and try to figure out what it does. See also the 1uF base capacitor, that in hard switching conditions could easily destroy the B-E junction, but mine is not emitter-hard-switching...
Actually, the first thing that I found out is that hard switching over the emitter was not going to produce the performance that I wanted. Paradoxically, with bipolar transistors the fastest turn-off behaviour with no current-tail is only obtained when the rate at which charge is extracted from the base is intentionally limited, and the optimum value seems to be somewhat smaller than Ic. In the shown working conditions I got no current tail at all as the emitter does not seem to get pulled up during the collector voltage rise period. I have to verify that for lower currents and lower forced gains, though.
Conrolled turnoff for HV bipolars is a pretty old concept, and was covered quite extensively in Philips and other application notes in the 80's, when HV bipolar transistors were the only economical alternative for switching power supplies. If you can dig them up, the app notes make for really interesting reading. I hadn't run into the concept in the context of emitter switching, but it makes sense in retrospect.
For those unfamiliar with the concept (not you, Eva...), there is an optimum rate of turnoff for HV bipolar transistors. This is mostly to do with the high resistivity collector region needed in order to get a high breakdown voltage in a bipolar transistor. Unless one somehow keeps the transistor out of saturation (not a good idea - high VCE drop), you need to fill the collector region with charge for a low VCE drop during turnon (forward bias the base-collector junction), then pull all that charge out during turnoff. If you try to withdraw charge too rapidly by driving the base-emitter junction too hard, some charge remains trapped in the collector region, resulting in a turnoff tail. If you don't drive the base hard enough, you get slow turnoff for the usual obvious reasons. Some designs used an inductor in series with the base to control the rate of turnoff. The inductor would also reverse avalanche the base-emitter junction. Motorola published an application note about this and concluded that reverse avalanching the BE junction did not harm the device, assuming a reasonable reverse current.
Eva, I would be interested to see the upper transistor base current during turnoff. It may be that the capacitor on the gate to drain of your bottom MOSFET is smearing out the base current pulse in the upper transistor during turnoff so that it is less likely to damage the device. BTW - you sure get a lot of mileage out of 13009s.....
For those unfamiliar with the concept (not you, Eva...), there is an optimum rate of turnoff for HV bipolar transistors. This is mostly to do with the high resistivity collector region needed in order to get a high breakdown voltage in a bipolar transistor. Unless one somehow keeps the transistor out of saturation (not a good idea - high VCE drop), you need to fill the collector region with charge for a low VCE drop during turnon (forward bias the base-collector junction), then pull all that charge out during turnoff. If you try to withdraw charge too rapidly by driving the base-emitter junction too hard, some charge remains trapped in the collector region, resulting in a turnoff tail. If you don't drive the base hard enough, you get slow turnoff for the usual obvious reasons. Some designs used an inductor in series with the base to control the rate of turnoff. The inductor would also reverse avalanche the base-emitter junction. Motorola published an application note about this and concluded that reverse avalanching the BE junction did not harm the device, assuming a reasonable reverse current.
Eva, I would be interested to see the upper transistor base current during turnoff. It may be that the capacitor on the gate to drain of your bottom MOSFET is smearing out the base current pulse in the upper transistor during turnoff so that it is less likely to damage the device. BTW - you sure get a lot of mileage out of 13009s.....
I have these Philips application notes about HV bipolars, they are worth reading.
What the capacitor across the MOSFET does is to force the drain voltage to have fixed rising and falling slopes. That in turn forces the 1uF capacitor to be charged and discharged through the base at constant rates, and thus, with constant currents.
My rates are currently 8V/us for turn-on and 6V/us for turn-off, yielding capacitor currents of 8A and 6A respectively. That's what I have empirically found to be optimum in my clumsy test fixture. For turn-off 6A may seem a small value, particularly because 2.4A from the current transformer have to be substracted from that, yielding a 3.6A real value, but the transistor likes that rate, it seems to have stopped conducting almost completely by the time the collector voltage starts to rise.
I've also found out by myself that the BE junction of bipolar transistors can whitstand reverse avalanche without causing any damage to the device as long as the pulse energy and total dissipation are kept under control. One of the undesirable features of my previous base drive system is that, when one set of transistors is turned on, it briefly drives the other set into reverse avalanche due to drive transformer leakage inductance. That's not destructibe but steals precious base current.
What the capacitor across the MOSFET does is to force the drain voltage to have fixed rising and falling slopes. That in turn forces the 1uF capacitor to be charged and discharged through the base at constant rates, and thus, with constant currents.
My rates are currently 8V/us for turn-on and 6V/us for turn-off, yielding capacitor currents of 8A and 6A respectively. That's what I have empirically found to be optimum in my clumsy test fixture. For turn-off 6A may seem a small value, particularly because 2.4A from the current transformer have to be substracted from that, yielding a 3.6A real value, but the transistor likes that rate, it seems to have stopped conducting almost completely by the time the collector voltage starts to rise.
I've also found out by myself that the BE junction of bipolar transistors can whitstand reverse avalanche without causing any damage to the device as long as the pulse energy and total dissipation are kept under control. One of the undesirable features of my previous base drive system is that, when one set of transistors is turned on, it briefly drives the other set into reverse avalanche due to drive transformer leakage inductance. That's not destructibe but steals precious base current.
The 47 ohm resistor that dissipates the energy stored in the inductor just got too hot and desoldered from the diode when I was testing at 12A. That killed the MJE13009 instantaneously, but after replacing it I noticed that this little transistor survives unclamped turn-off at low current levels 
That capture shows a nearly 1.000 Volt collector turn-off spike. Inductor current was 4.5A before turn-off, but most of that energy is transferred to the capacitances (and to the resistivity of the own iron-powder core) instead of avalanche dissipated in the bipolar.
Note how the transistor becomes slower at low Ic levels.
That capture shows a nearly 1.000 Volt collector turn-off spike. Inductor current was 4.5A before turn-off, but most of that energy is transferred to the capacitances (and to the resistivity of the own iron-powder core) instead of avalanche dissipated in the bipolar.
An externally hosted image should be here but it was not working when we last tested it.
Note how the transistor becomes slower at low Ic levels.
Updated circuit:
To my surprise, connecting the 560pF capacitor that way not only solves all the MOSFET parasitistic oscillation issues that I previously had, but also produces better waveforms. It compensates for high B-E impedance during turn-on, that otherwise would reduce the amount of base boost current during the first 200ns, and it also allows for better B-E reverse biasing at turn-off.
I also rearranged the current sense transformer to avoid persistent base drive after collector current has ceased. It should have been that way from the beggining. Forced gain is now 4, as it produces smaller conduction losses without degrading turn-off performance too much.
This is how Vbe looks during a switching period at Ic=11A on a resistive (and slightly inductive) load. It was measured across transistor legs to avoid breadboard resistance. Scales are 1V/div and 1us/div. The 0V level corresponds to that small horizontal red mark on the left border of the grid. Note the increased Vbe due to 8A of Ib boost during the first microsecond, the progressive charge removal at the end of the cycle, and the convenient B-E reverse biasing during Vce rise. Some inductive artifacts are also shown.
And this is how Vce looks in the same conditions and measured directly across transistor legs. Note the dramatic effects of those 8A of base drive boost applied during the first microsecond, that seem to eliminate dynamic saturation effects almost completely, and the ultra-fast desaturation process.
I'm seriously considering that topology
An externally hosted image should be here but it was not working when we last tested it.
To my surprise, connecting the 560pF capacitor that way not only solves all the MOSFET parasitistic oscillation issues that I previously had, but also produces better waveforms. It compensates for high B-E impedance during turn-on, that otherwise would reduce the amount of base boost current during the first 200ns, and it also allows for better B-E reverse biasing at turn-off.
I also rearranged the current sense transformer to avoid persistent base drive after collector current has ceased. It should have been that way from the beggining. Forced gain is now 4, as it produces smaller conduction losses without degrading turn-off performance too much.
This is how Vbe looks during a switching period at Ic=11A on a resistive (and slightly inductive) load. It was measured across transistor legs to avoid breadboard resistance. Scales are 1V/div and 1us/div. The 0V level corresponds to that small horizontal red mark on the left border of the grid. Note the increased Vbe due to 8A of Ib boost during the first microsecond, the progressive charge removal at the end of the cycle, and the convenient B-E reverse biasing during Vce rise. Some inductive artifacts are also shown.
An externally hosted image should be here but it was not working when we last tested it.
And this is how Vce looks in the same conditions and measured directly across transistor legs. Note the dramatic effects of those 8A of base drive boost applied during the first microsecond, that seem to eliminate dynamic saturation effects almost completely, and the ultra-fast desaturation process.
An externally hosted image should be here but it was not working when we last tested it.
I'm seriously considering that topology
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