Trimming Voltage Control Circuit

[dave_robinson_022]

I am using the circuit shown to control voltage for a bench power supply build.

It will be used in conjunction with a current control circuit which is why it pulls MOSFET gate low instead of driving it.

the MOSFET is IRFP064N and I have tested it to require below 1.907V to turn off.

However as it stands I can't get the gate voltage below 2.553V. I did build a circuit using two TLE2141 which have null pins, and with offset trimming of the integrator op amp (using 5k trimpot and 1k to -12V) I got it to 2.459V which still has about 67mV output.

What is the best way to trim this circuit and at which point?

[Terry Bites]

I cant quite figure out the function of the zener. What voltages dow you see with the sense inputs shorted and a fixed voltage on the DAC input.
Trimming the opamp isnt going to make much difference if there is an unkown circuit error. Is the LT1054 generating sufficient current for the ngative rail.
The opamp via the diode should be able to pull the gate least -8V under normal circumstances. Have you checked the stabilty of the control loop?

[dave_robinson_022]

I took the design from Peter Oakes design here



I designed and made up some PCBs for it but was having real issues, so I decided to go back to start from scratch.

I understood the Zener Diode reduces pull up voltage down because MOSFET turns full on at around 4V so I kept it the driving stage the same.

I took out the voltage sensing Op Amp because I don't need it and removed the buffer because I am using an MPC3428 ADC and it can read negative voltages (I thought one less source of error)

This option also allows me to use one TLE2442 per control circuit (voltage and current)

With the sense inputs shorted I get 6.7V. (I have not been using a DAC in this test circuit, just controlled voltage from an divider circuit)

The LT1054 should provide 100mA from the data sheet and the Op Amp is drawing far less.

I haven't checked the stability of the control loop yet as I have been at home and not had a scope.

Does this make it clearer and suggest anything else I may be doing wrong?

[dave_robinson_022]

I took some further voltage readings but still need to check with scope tomorrow:

With a set point of 0V I have the following relative to analog gnd

200 mV across sense inputs
-6.6mV out of first Op Amp which is spot on with a gain of 0.033

0V going into + of second Op Amp from summing junction
0V at - of second Op Amp
2.052V at output of second Op Amp?

This gives a gate voltage of 2.565V

[741]

Suppose we had a standard integrator: + input = 0V and - input also 0V, let it settle.

Now put the - input at -3V: the integrator must ramp up at a steady rate in order to supply a steady current (and thus a steady 3V at the - input).

Now, when the output hits 2.052V, suddenly change the + pin (non-inverting input) to 0V.

The output must freeze so that no current flows through the 100R resistor. If the output does anything else but stay fixed, current will flow.

[dave_robinson_022]

I will give that a go now I am back in and can hook up the scope.

I went through it again and rechecked. As I noticed above I was getting the -6.6mV out of the first Op Amp but after going through the 10K resistor I was getting 0V at the + input to the second Op Amp.

So I removed this resistor, gate is now pulled down but oscillating (on DMM anyway) between -4.442 and -4.4450.

It is now 5.1mV across sense resistors

[dave_robinson_022]

That did not work, as it no longer adjusts.

I hooked scope up to gate output at last. It is unstable. With 10K resistors on setpoint and output from first Op Amp there is now approx 290mV output oscillating on gate 145mV Peak to Peak.

I changed these resistor values and found that replacing with 1k gave 162mV output but oscillates 400mV and changing to 100R gives 150mV output but at 450mV oscillation?