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Old 26th March 2014, 01:40 AM   #1
BRSHiFi is offline BRSHiFi  United States
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Default HV Logic Control Approach

My next project is a logic controlled high voltage power supply. My goal is to regulate 0-500V at 100mA of with the control of a programmable voltage reference. The circuit in the posted schematic shows a solid state unit.

The circuit in spice oscillates in a weird manner. I simulated a tube equivalent that is stable and ripple free with low currents. I want to build the SS as I will be building more than one of these power supplies.

The circuit is a conventional error-amplifier based regulator with a high-voltage transistor driven by an op amp. V5 represents the programmable reference.

I tried to place a cap on the output. What is interesting is that the oscillation is suppressed, but not eliminated. It also changes frequency depending upon the set voltage and the value of the cap.

I'm wondering, is this the best circuit for the job? If not are their any devices better suitable for this task anyone knows of?
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Old 26th March 2014, 01:22 PM   #2
Ketje is offline Ketje  Belgium
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Probably to much loop gain especialy in high frequenties.You could try 1k between opa and Q1+ 1nF c to b.
Did you see the dissipation in Q1 ? Halfway gives 250V 25mA = 6.25W !
Better use a mosfet pass-transistor, no need of only 10k but much larger.
Mona
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Old 26th March 2014, 03:24 PM   #3
BRSHiFi is offline BRSHiFi  United States
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Placing the cap suppresses and changes the oscillation but does not eliminate it. Here is the simulation along with a vacuum tube version juxtaposed. The tube version works in simulation. Here is the LTspice project.
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Old 26th March 2014, 04:47 PM   #4
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Here it is, with the necessary added parts for compensation and proper operation.

R3 & C3 compensate the op-amp, otherwise this will follow an endless "tail-chase" with the relatively slow output stage.

C1 feeds more of the output ripple into the loop, for nulling.

In the output stage: removed the cap from there (it is slow enough as it is). Diode added for protection, and for some subtle "push-pull", if the load throws energy back at the supply (which it will!).

I stabilised and linearised the output stage gain with R1, which must be carefully selected in each case.

Common-mode input voltage range of the opamp will restrict the range of outputs to about 200V to 450V in this case.

Lastly, I have added the load, and an output cap (you can't do without these).

Not shown: Needs protection diodes on the input & output of the opamp (eg 5.6V zener)

Sadly, these kind of regulators do not sound very good, especially if used for a power amp.
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Old 26th March 2014, 05:25 PM   #5
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Quote:
Originally Posted by Rod Coleman View Post
...

Sadly, these kind of regulators do not sound very good, especially if used for a power amp.
Can you elaborate on that?
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Old 26th March 2014, 05:28 PM   #6
BRSHiFi is offline BRSHiFi  United States
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The design is not going in a power amplifier.
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Old 26th March 2014, 05:40 PM   #7
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Can you elaborate on that?
There are many weaknesses, but the important ones:

- Huge power FET driven by tens-to-hundreds of KΩ is relatively slow. Its settling time and transient behaviour will be relatively poor. The rising and falling edges of the output voltage in transient loads is notably asymmetrical.

- In power amps, the reactive load (speakers) will throw energy back at the dc supply. The regulator can do little about this unless the output rises by 600mV, and the diode turns ON.

Reliability wise - beware that power FETs and opamps are VERY unforgiving of overvoltage transients, even those you never see.
1000V rated FETs, and protection diodes on everything are needed.

Having said all that, if the output wiring is kept short (inductance is unhelpful), and the load current does not change dramatically, it may be OK.
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Old 26th March 2014, 07:22 PM   #8
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Originally Posted by Rod Coleman View Post
There are many weaknesses, but the important ones:

- Huge power FET driven by tens-to-hundreds of KΩ is relatively slow. Its settling time and transient behaviour will be relatively poor. The rising and falling edges of the output voltage in transient loads is notably asymmetrical.

- In power amps, the reactive load (speakers) will throw energy back at the dc supply. The regulator can do little about this unless the output rises by 600mV, and the diode turns ON.

Reliability wise - beware that power FETs and opamps are VERY unforgiving of overvoltage transients, even those you never see.
1000V rated FETs, and protection diodes on everything are needed.

Having said all that, if the output wiring is kept short (inductance is unhelpful), and the load current does not change dramatically, it may be OK.
Thanks! Are you refering to R9 as the driving resistor?

I'd personally increase R4 and R6. At 500V, the 47k needs te be of some serious power rating.
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Old 26th March 2014, 07:30 PM   #9
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Originally Posted by funk1980 View Post
Thanks! Are you refering to R9 as the driving resistor?

I'd personally increase R4 and R6. At 500V, the 47k needs te be of some serious power rating.
Yes, R9 forms a pole with the (effective) gate capacitance. Minimise the Cin by keeping 25V or more across the FET.

Of course, increase the feedback resistors to suit your output voltage. This works better than altering the reference voltage (opamp common-mode input range restriction).

The upper feedback resistor needs to be high voltage rated, too. Or use two parts in series.
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Old 7th April 2014, 11:11 PM   #10
BRSHiFi is offline BRSHiFi  United States
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Not shown: Needs protection diodes on the input & output of the opamp (eg 5.6V zener)
Where should I place these diodes?
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