Circuit operation

I have the below input protection and filter circuit. I want to understand how it works.



Can someone tell me the purpose of R0101 and D0102 and D0104?

As far as my understanding level, when the voltage at the emitter of Q0102 exceeds the voltage at the base of Q0102 by its base emitter voltage threshold (assumed to be 0.6V), the Q0102 transistor will short the gate and source of the MOSFET and turn it OFF. So, when the input voltage goes low, the input gets cut off. Am I correct? Is that the intended purpose?

The Zener diode is to protect the gate-source of the MOSFET not to exceed the maximum GS Voltage. The Pi Filter, C0102, L0101 and C0103 is there to reduce the conducted emission from the U12V to the input rail.

Can someone explain the purpose of R010, D0102 & D0104, please?

Hi,

Wrong guesses:

D0102 = I have no idea.
D0104 might be temperature compensation for the Zener.
The backwards PMOS could be for reverse-polarity protecion.
Q0102 looks like it conducts reverse voltage/current to the gate of Q0101 in a reverse-polarity event, ensuring it is off. If Vin is 0V, Q0102 should conduct; if Vin is 12V, Q0102 should be off, allowing Q0101 gate to be at about 0.6V and so conduct.

I'm probably wrong about everything. You could look for temperature-compensated Zener references and reverse-polarity protection circuit application notes, in case they help.

Hi,

Please give a link to the source of the schematic.

Klaus

Something doesn't look right, please do as Klaus asks.

The 'U12V' is clearly DC but what is the input? If it was DC the circuit can only work as a filter because the MOSFET substrate diode will always conduct. The R0101/D0102 looks like it is to dampen oscillations in the inductor but it would do nothing if the input was DC and in any case the capacitor values seem disproportionate to the inductor value unless the input was some strange mix of LF and HF signals.

Is something else connected to the junction of D0104/R0104 ?

Brian.

I agree with Brian; something looks terribly amiss with this circuit. You've got a straight DC path through R0101, D0102 (paralleled with L0101) and Q0102's intrinsic diode.

You call this a "protection circuit". Who, or what, is it protecting, and from what?

The schematic isn't necessarily wrong, but the operation is completely misunderstood. Q0101 is working as reverse voltage protection.

To study the anatomy of electronic circuits, you need to practice dissection. This is how I see it.

Consider the part left of Q0101. This part can be studied independently of what is happening on the right side. This is a block that can be studied easily.

Surely you have understood the role of D0101; it prevents voltages above 12V or voltages below 12V to be applied at the input. This number depends on the rating of the zener diodes and it may even be +/-15V because the input is labeled at 12V. It serves no other purpose. If the input is +12V<U<-12V, this has no role.

The four capacitors with the inductor forms a basic pi filter. The electrolytic capacitors are in parallel with ceramic capacitors for fast response.

What does the pair R0101 and D0102 doing here?

Under steady state, the voltage across the inductor is zero. Then the diode does not conduct.

If the load (Q0101 on) takes an appreciable current, the D0102 conducts and bypasses partly the filter. So D0102 must be fully rated for the load current.

If the input voltage is >-12V but less than 0, the diode D0102 does not conduct but the inductor must pass the full current. However, Q0101 blocks current.

So the role of the circuit left of Q0101 is:

1. blocks input above 12V or below -12V.

2. Filters noise or acts as a low pass filter.

3. When fast switching or large currents are needed, R0101 and D0102 comes into play but only if the input voltage is >0.6V (diode drop).

4. The pass transistor Q0101 prevents negative voltage.