How should I monitor the output voltage of a boost converter?

[dann11]

How should I monitor the output voltage of a boost converter?

for example I have a boost converter with output voltage of 60V +/- 5Vand I have a voltage sense that sense the change of these value, should my voltage sense monitor the levels above the its maximum or below its minimum level? anyway the voltage sense is used to switch the PWM controller on and off.

 

[BradtheRad]

Obviously a 60V reference is unavailable at powerup. Therefore you must arrange things so that switching occurs continuously, regardless of output level. (This is a quirk of boost converters.)

Divide down the 60V so it creates a usable feedback signal. Options might be an optocoupler, and/or zener diode to create expanded scale measurement.

You must start operating at a duty cycle that you have found is guaranteed to result in 60V output. Eventually you want feedback to start its action when output is in the vicinity of 60V. Therefore make needed design adjustments in gain, threshold, envelope, etc.

 

[Warpspeed]

As Brad says, fit a voltage divider from the final output voltage point to provide negative feedback.
Use the soft start feature on the PWM chip at initial turn on to slowly raise the output up to the 60v set point where it should then stay.

 

[treez]

OP are you doing ON/OFF control of the boost converter, or are you controlling the vout with an error amplifier?

 

[KlausST]

Hi,

Just for ON/OFF I'd use a comparator, a voltage reference (if you need precision) and some resistors.
With this I build a comparator with hysteresis.

Klaus

 

[dann11]

So the control feedback must monitor the maximum value of the boost output? right? must it be to protect the inductor from saturation?

 

[Vbase]

The inductor must not saturate under any conditions because that can destroy the switching transistor.
The inductor value is selected to be charged below its max current when it conducts for the full period of the PWM and max input voltage. The inductor current depends on input voltage and not the feedback.
The feedback should be like Klaus suggested and independent of the coil.

 

[Warpspeed]

So the control feedback must monitor the maximum value of the boost output? right?

The feedback is to set the output voltage to the 60 volts that you want.

 

[treez]

Here is how smps controls its output voltage (this is in current mode control).
Download free ltspice and run this, and you see the regulation components in the flesh.
Ok it’s a flyback converter but the feedback circuitry is generally the same for the boost

 

[Vbase]

Download free ltspice and run this, and you see the regulation components in the flesh.

'In the flesh' is exactly the opposite of simulation.

 

[treez]

true but for the purpose of gaining the understanding, the sim is absolutely fine.