It is four hundred and seventy five amperesPafi said:
http://www.diyaudio.com/forums/showthread.php?postid=1484597#post1484597
RiskCord said:
..as accurate as possible?
How about trying as accurate as necessary?
It is allowed to neglect quite a lot of things, but I would like to highlight two crucial points that are most often not simulated, but tend to show their dragon face in reality.
- The parasitic layout inductances and the parasitic inductances of the devices.
Nice start for TO-220 is to simulate roughly 5nH in every leg in order to include the unavoidable geometric properties of that package. Also put about 0.5nH inductance for every mm copper trace in your high current paths (MosFets, free wheeling diodes, rail caps....).
- Your diode models should incoporate the reverse recovery characteristic.
And then simulate hard switching conditions, means go for output currents above the filter ripple current... Force PSPICE to go for small time steps (<10ns), otherwise you might miss the clue...
As accurate as necessary... I like that better.
Yes, I included the parasitic inductances, the non-linear Gate-Drain and Drain-Source capacitances and I made a model representing the reverse recovery characteristics of the diode. The waveforms looks a bit ugly, so I guess the simulation is working just fine
Thanks for the advice on the inductance of PCB traces. I didnt know how to estimate them so I just doubled the values of the MOSFETs inductances. Anyway, I'm just about to start the PCB design and I guess I'll upload my design here so you can have some fun criticizing it
By the way, I'm not using PSpice. Instead, NL5 is my choice. It works on piece wise linear approximation which is quite suitable for non-linear applications as it is extremely fast and doesn't have (usually) convergence issues.
Yes, I included the parasitic inductances, the non-linear Gate-Drain and Drain-Source capacitances and I made a model representing the reverse recovery characteristics of the diode. The waveforms looks a bit ugly, so I guess the simulation is working just fine
Thanks for the advice on the inductance of PCB traces. I didnt know how to estimate them so I just doubled the values of the MOSFETs inductances. Anyway, I'm just about to start the PCB design and I guess I'll upload my design here so you can have some fun criticizing it
By the way, I'm not using PSpice. Instead, NL5 is my choice. It works on piece wise linear approximation which is quite suitable for non-linear applications as it is extremely fast and doesn't have (usually) convergence issues.
Pabo said:
I remember attending a technical presentation by one of the major MOSFET manufacturers (IR, Fairchild? don't remember) that explained this very well. They made the point that the only way to achieve the intended results was to co-package the Fet and the schotkky.
Of course, they also had in the market Fet-schottky devices they were trying to sell...
Hello,
I am new here in this forum and I am very interested in this topic.
An interesting alternative to the shottky diode is a
- voltage control
- of the output voltage of the freewheel FET
- during the transition between the
- freewheel phase and the
- switching phase
If you
- control the output voltage to a value, e.g. 0.6V
- the intrinsic diode of the freewheel FET
- will not become conductive.
I attached some waveforms. If anybody is interested, I will give much more explanations.
Greetings,
Matthias
P.S.: I was posting something before. Please consider this as a draft. I was pushing the Submit Reply Bottom unintentionally.
I am new here in this forum and I am very interested in this topic.
An interesting alternative to the shottky diode is a
- voltage control
- of the output voltage of the freewheel FET
- during the transition between the
- freewheel phase and the
- switching phase
If you
- control the output voltage to a value, e.g. 0.6V
- the intrinsic diode of the freewheel FET
- will not become conductive.
I attached some waveforms. If anybody is interested, I will give much more explanations.
Greetings,
Matthias
P.S.: I was posting something before. Please consider this as a draft. I was pushing the Submit Reply Bottom unintentionally.
Attachments
"What part #/manufacturer is that FET/Schottky? The IXYS one is very difficult to get."
Fairchild has a series of MOSFET/Schottky combos but their all for low Vds. (< 60v)
Microsemi has its MOSKEY series, they go up to 100 v (MSAER57N10A) but it's the kind of custom package that Eva would hate
And finally, IRF has its Fettky series (cool name, isnt it?!), but again, low voltage applications.
Obviously the reason for this is that physically it is not an easy task to develope schottky diodes with a large breakdown voltage. In fact, discrete schottky diodes with Vbd > 100 are hardly available.
On the other hand, for this range of Vds I think you can get conventional MOSFETs with very nice body diode specs. In any case, if time and money is available, it wouldn't hurt to try both and compare performances.
Regards
Pablo
Fairchild has a series of MOSFET/Schottky combos but their all for low Vds. (< 60v)
Microsemi has its MOSKEY series, they go up to 100 v (MSAER57N10A) but it's the kind of custom package that Eva would hate
And finally, IRF has its Fettky series (cool name, isnt it?!), but again, low voltage applications.
Obviously the reason for this is that physically it is not an easy task to develope schottky diodes with a large breakdown voltage. In fact, discrete schottky diodes with Vbd > 100 are hardly available.
On the other hand, for this range of Vds I think you can get conventional MOSFETs with very nice body diode specs. In any case, if time and money is available, it wouldn't hurt to try both and compare performances.
Regards
Pablo
Hi, Pablo,
You're right, in low voltages the new mosfets are already good. The problem is for 300-400V mosfets. IXYS one is very good, but digikey don't stock them.
I wonder if there are other manufacturer that produce 300-400V mosfet+series schottky+parrarel ultrafast dioda in one package, and "buyable"
MIAS' method is interesting. I just wanted to know what happens with this technique if the current in the inductor is 10's of ampere headed to base of the totempole's predriver (only mA here). Will it ruin the predriver or stage before predriver?
You're right, in low voltages the new mosfets are already good. The problem is for 300-400V mosfets. IXYS one is very good, but digikey don't stock them.
I wonder if there are other manufacturer that produce 300-400V mosfet+series schottky+parrarel ultrafast dioda in one package, and "buyable"
MIAS' method is interesting. I just wanted to know what happens with this technique if the current in the inductor is 10's of ampere headed to base of the totempole's predriver (only mA here). Will it ruin the predriver or stage before predriver?
Hello lumanauw,
I made this circuit in my vacancy. The really funny thing is: It looks risky, but it is not. The thing is a
- MOSFET
- can get conductive in reverse mode
- below its threshold voltage.
This why you can control a MOSFET with a gate voltage very little above the threshold voltage. And this is why during the whole transistion a bridge short circuit is safely prevented. As one can see in the graph - when the MOSFET gets into forward mode the gate-source voltage is below the treshold voltage.
This capacitor has the effect to eleminate the influence of the miller capacitor. In reverse mode this capacitor can lead to a ringing tendency. The gain of the control circuit is high, can be decreased with a resistor between the emitter of the npn transistor in the totempole circuit and GND. But this has then other problems - I tried it.
I did not try so many AMPs. Then also the stray inductances may play a role. Nevertheless, the MOS-Drive circuit is safe from being damaged. The Push Pull Circuit has to be low ohmic in any case, otherwise you will get some funny steering of the Freewheel FET in linear mode also, when you use a conventional MOS Drive.
Greetings,
Matthias
--------------------------------------
Hello Pafi,
Yes, but the MOSFETs are getting better and better, the RDSOns are getting smaller and smaller... And also with 0.7V control there is an effect. The voltage drop of the intrinsic diode in the amp range is pretty high.
Greetings,
Matthias
--------------------------------------
Hello classdphile,
I am really very interested, whether this circuit is already state of the art. Do you or anybody here have any idea, where I can find this out?
Greetings,
Matthias
I made this circuit in my vacancy. The really funny thing is: It looks risky, but it is not. The thing is a
- MOSFET
- can get conductive in reverse mode
- below its threshold voltage.
This why you can control a MOSFET with a gate voltage very little above the threshold voltage. And this is why during the whole transistion a bridge short circuit is safely prevented. As one can see in the graph - when the MOSFET gets into forward mode the gate-source voltage is below the treshold voltage.
This capacitor has the effect to eleminate the influence of the miller capacitor. In reverse mode this capacitor can lead to a ringing tendency. The gain of the control circuit is high, can be decreased with a resistor between the emitter of the npn transistor in the totempole circuit and GND. But this has then other problems - I tried it.
I did not try so many AMPs. Then also the stray inductances may play a role. Nevertheless, the MOS-Drive circuit is safe from being damaged. The Push Pull Circuit has to be low ohmic in any case, otherwise you will get some funny steering of the Freewheel FET in linear mode also, when you use a conventional MOS Drive.
Greetings,
Matthias
--------------------------------------
Hello Pafi,
Yes, but the MOSFETs are getting better and better, the RDSOns are getting smaller and smaller...
And also with 0.7V control there is an effect. The voltage drop of the intrinsic diode in the amp range is pretty high.Greetings,
Matthias
--------------------------------------
Hello classdphile,
I am really very interested, whether this circuit is already state of the art. Do you or anybody here have any idea, where I can find this out?
Greetings,
Matthias
Attachments
Hi, MIAS,
I have a dumb question. In your drawing, you don't use external dioda (parrarel to the body dioda).
If the body dioda is already disabled with your method. The upper mosfet is already off, but the lower mosfet is not on yet, the left side of the inductor will instantly change polarity 180deg, where will this energi inside the inductor go to, since the body dioda is already disabled, but there is no external dioda (parrarel with body dioda's position)?
I have a dumb question. In your drawing, you don't use external dioda (parrarel to the body dioda).
If the body dioda is already disabled with your method. The upper mosfet is already off, but the lower mosfet is not on yet, the left side of the inductor will instantly change polarity 180deg, where will this energi inside the inductor go to, since the body dioda is already disabled, but there is no external dioda (parrarel with body dioda's position)?
Hello lumanauw,
One cannot disable the intrinsic diode. All you can try is to get this part of the freewheel FET not conductive. In the transistion phase
- when the upper FET is switching off
- and the regulator circuit is not reacting so fast,
the only thing what is happening is that the diode could become conductive, before the freewheel FET is switched
- either hardly
- or the regulation is reacting.
Hello Pafi,
There are 3.9mOhm, D2-Pack, 100V FETs on the market. The name is IPB04CN10N G:
http://www.infineon.com/dgdl/IPP04CN10N_Rev1.2.pdf?folderId=db3a3043156fd5730115c7d50620107c&fileId=db3a3043156fd5730115c7df60f4108a
Let us asume,
- we go up to 125deg
- we have here an RDSon of 8mOhm
- and we control to 0.4V.
Then we control a current of
- 0.4V/8mOhm
- 50A at high temperatures
When we assume a supply voltage of
- 60V.
We can control a power of
- 3kW with here.
In the circuit diagram I sketched the MOS-driver is pretty lowohmic. It must only be active when the Low Side Transistor cannot generate any switching slopes, etc.. In case of the low side transistor gets in the role of the switching FET, then you have to steer on the FET more high-ohmic, i.e. with a defined gate resistor.
Nevertheless, I strongly recommend in any case to do this, also in case of I do not want any control. In case of you use the FET as Freewheel FET always take care you have a low ohmic - I mean a very low ohmic - current sink. Otherwise you will charge up your gate and steer the Freewheel FET in the linear region during the transistion phase, which makes neverending troubles.
Of course, in the phases where the FET gets forward conductive, you cannot switch OFF the gate without preresistor.
Greetings,
Matthias
One cannot disable the intrinsic diode. All you can try is to get this part of the freewheel FET not conductive. In the transistion phase
- when the upper FET is switching off
- and the regulator circuit is not reacting so fast,
the only thing what is happening is that the diode could become conductive, before the freewheel FET is switched
- either hardly
- or the regulation is reacting.
Hello Pafi,
There are 3.9mOhm, D2-Pack, 100V FETs on the market. The name is IPB04CN10N G:
http://www.infineon.com/dgdl/IPP04CN10N_Rev1.2.pdf?folderId=db3a3043156fd5730115c7d50620107c&fileId=db3a3043156fd5730115c7df60f4108a
Let us asume,
- we go up to 125deg
- we have here an RDSon of 8mOhm
- and we control to 0.4V.
Then we control a current of
- 0.4V/8mOhm
- 50A at high temperatures
When we assume a supply voltage of
- 60V.
We can control a power of
- 3kW with here.
In the circuit diagram I sketched the MOS-driver is pretty lowohmic. It must only be active when the Low Side Transistor cannot generate any switching slopes, etc.. In case of the low side transistor gets in the role of the switching FET, then you have to steer on the FET more high-ohmic, i.e. with a defined gate resistor.
Nevertheless, I strongly recommend in any case to do this, also in case of I do not want any control. In case of you use the FET as Freewheel FET always take care you have a low ohmic - I mean a very low ohmic - current sink. Otherwise you will charge up your gate and steer the Freewheel FET in the linear region during the transistion phase, which makes neverending troubles.
Of course, in the phases where the FET gets forward conductive, you cannot switch OFF the gate without preresistor.
Greetings,
Matthias
mias!
An amplifier is used to drive speakers. Speakers have a given impedance, so you can't multiple any currents and voltages to determine output power!
We prefer single-ended topology. +/-40 V allowes 200 W @ 4 ohm.
If we stay below 100 V, the body-diode is not a serious problem, there are (cheap and low Qg!) MOSFETs with very low stored charge, like IRF540Z, FDP3652, etc... But 100 V is sometimes not enough.
There are some 150 V MOSFETs with low enough Qrr, but saidly it's almost impossible to buy them here.
The real problem starts at 200 V. (Read the first post of this topic!)
I don't understand this! High/low ohmic? How could this solve the problem? MOSFET needs completely different gate-control depending on the direction of drain-current, because at positive drain-current your control-loop turns to positive feedback, and switches off. In an amplifier this happens in every cycle at idle (and at low signal levels), but you mustn't allow switching off just because the current changes its polarity, FET have to stay ON till the next half-cycle!
An amplifier is used to drive speakers. Speakers have a given impedance, so you can't multiple any currents and voltages to determine output power!
We prefer single-ended topology. +/-40 V allowes 200 W @ 4 ohm.
If we stay below 100 V, the body-diode is not a serious problem, there are (cheap and low Qg!) MOSFETs with very low stored charge, like IRF540Z, FDP3652, etc... But 100 V is sometimes not enough.
There are some 150 V MOSFETs with low enough Qrr, but saidly it's almost impossible to buy them here.
The real problem starts at 200 V. (Read the first post of this topic!)
I don't understand this! High/low ohmic? How could this solve the problem? MOSFET needs completely different gate-control depending on the direction of drain-current, because at positive drain-current your control-loop turns to positive feedback, and switches off. In an amplifier this happens in every cycle at idle (and at low signal levels), but you mustn't allow switching off just because the current changes its polarity, FET have to stay ON till the next half-cycle!
Hello Pafi,
I might be wrong, because I did not try it up to now. But let me give a chance to explain.
My circuit is only active, when this signal at the entry of the totem pole stage is high because then - and only then - a current can flow through this small signal diode that closes the control lope.
When you want to steer ON the FET in forward mode, you have to put the entry signal of the totem pole drive to zero, and then this small signal diode is no longer conductive and will have no influence.
This means the circuit I propose is only active when you try to switch OFF the FET. And even then, it is only active in reverse mode. When you have a positive drain source voltage the current through the small signal diode will become zero because of the resistor in series. All the current which is coming from the input signal and the resistor between the input signal and the base of the bipolar transistor at the entry of the totem pole stage is flowing in the base of this transistor.
There is only a problem with the switching slopes. You might get a too fast di/dt with this gate drive circuit when you are operating the transistor in forward mode. Neverheless, in forward mode the control loop cannot be active. It will only operate in reverse mode.
Greetings,
Matthias
P.S.: My circuit is just for substituting the shottky diode in parallel to the FET, means for realtivly low voltages. When you have a real high voltage operation you can oly get rid of the storage charge effect by
- two diodes
- one in the same current path of the Freewheel FET
- but this one has antiseriell direction to the intrinsic diode of the FET.
- the other one parallel to the the last described diode and the the Frewwheel FET.
- the direction of this diode is same as the direction of the intrinsic diode.
Thiis solution is common.
Of course, when the RDSON*I is higher than 0.5V, you can forget about my idea. Nevertheless, MOSFETs will become better and better. And to dope silicon with gold to achive a fast recovery is not easy from the point of reliablity.
I might be wrong, because I did not try it up to now. But let me give a chance to explain.
My circuit is only active, when this signal at the entry of the totem pole stage is high because then - and only then - a current can flow through this small signal diode that closes the control lope.
When you want to steer ON the FET in forward mode, you have to put the entry signal of the totem pole drive to zero, and then this small signal diode is no longer conductive and will have no influence.
This means the circuit I propose is only active when you try to switch OFF the FET. And even then, it is only active in reverse mode. When you have a positive drain source voltage the current through the small signal diode will become zero because of the resistor in series. All the current which is coming from the input signal and the resistor between the input signal and the base of the bipolar transistor at the entry of the totem pole stage is flowing in the base of this transistor.
There is only a problem with the switching slopes. You might get a too fast di/dt with this gate drive circuit when you are operating the transistor in forward mode. Neverheless, in forward mode the control loop cannot be active. It will only operate in reverse mode.
Greetings,
Matthias
P.S.: My circuit is just for substituting the shottky diode in parallel to the FET, means for realtivly low voltages. When you have a real high voltage operation you can oly get rid of the storage charge effect by
- two diodes
- one in the same current path of the Freewheel FET
- but this one has antiseriell direction to the intrinsic diode of the FET.
- the other one parallel to the the last described diode and the the Frewwheel FET.
- the direction of this diode is same as the direction of the intrinsic diode.
Thiis solution is common.
Of course, when the RDSON*I is higher than 0.5V, you can forget about my idea. Nevertheless, MOSFETs will become better and better. And to dope silicon with gold to achive a fast recovery is not easy from the point of reliablity.
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