hv reg setting the voltage

Help! I have no idea whats going on and would to love to know whats happening here so I can change the voltage. This is the HV section of the PM660 which I would love to repurpose for something else.

D7 is a zener diode used as a reference. Q3 compares this with the HT voltage via pot divider R7/R6 and all the rest of it closes the loop and provides some protection. The easy way to change the output voltage is by changing the ration of R7/R6.

Cheers

Ian

Awesome where can I find the equation for that? I've been looking for a while...

To a close approximation:

Vout = Vz (R6+R7) / R6

Where Vz is the zener voltage.

It's easy to see the circuit action around the Zener and Q3.

On one hand, you can see the base voltage (Vb) of Q3 is just VQ4source*R6(R6+R7).  However you also have the action around Q3, which is that the base voltage Vb should be Vzener + Vbe.  You can just set these equal for a good approximation.

Using the values in the schematic, and a 100V Zener, then Vb should sit at 100V + 6V (Vbe) = 106V.  So 106V = Vsource * R6 / (R6+R7), or 106V * (150K + 221K)/(150K) = Vsource, or Vsource = 262V.  This is the same approximation that Merlin provided, with the Vbe quantity added in. 

Awesome thanks for the info... was exactly what I need although I would love to know how the rest of the "protection" part of the circuit works. As for the continuation of the circuit do r10 and r9 form a voltage divider for the q5?

In the above cct the main protection is provided by D9. If the 245V output gets shorted, the remaining charge on C9 is a reverse bias to the FET and destroy it. D9 limit this to a diode drop.

D8 in the first circuit fulfils the same function.

Cheers

Ian

D10 also protects the IFR840 in short circuit conditions:  the maximum gate-source on these transistors is about 15-25V (to prevent punch-through), so the Zener will turn on in a dead short and drag the source down with it to protect the output transistor.

Better solution.
The mosfet has internal diode, so D9 is useless.
Add gate voltage filtering using RC.
I use 1Meg for the red resistor and 4.7uF and 2n2 in parallel
Output capacitor can be with lower value
also the input 100nF is useless too.

The orange resistor should be 1W or 2W with a value of 220-470ohm
This resistor and and R11 should be placed near the mosfet's pins

Nikolay said:

Better solution.

Click to expand...

To what problem though?  This is a post-regulation regulator:  the input voltage will already be pretty clean.  Adding another cap and resistor probably isn't going to change ripple very much and only add complexity.  Is the intent with the series resistance to try and limit fault current?

Agreed on the diode however.

The complexity is only 2 components: one resistor and a small capacitor. They will reduce all of the noise to the gate through R11 and gives clean noise free control voltage. Without these two components there can be ripple voltage, some hf noise or noise from the resistor divider too. Just two components will prevent everything. Their values can be optimized for the specific requirement.

330 ohm resistor has few benefits:
*It acts as a RC filter through the HV path . Imagine that the next part after the resistor is a capacitor.
*Limits the current
*Prevent oscillating ( this I read from Merlin Blenkowe's book)
Both 330ohm and R11 should be placed near the mosfet.

I might as well throw in my 2 cents.

The gate-source zener is a 20V device, which is pretty borderline. I'm not sure the gate-channel insulation would survive. I would replace it with a lower voltage device, say 10 or 12V.

The MOSFET has no protection against inrush current to C9 at start up. OK, there is a preregulator which will slow things down, but even so, I would add a 10R source resistor to limit the peak current to about 500mA which is within the SOA of the MOSFET.

As Nikolay mentions, an extra smoothing cap is a nice addition, though not essential. I wouldn't bother with an extra series resistor since the source resistance of the potential divider is already about 80k, which is plenty high.

Hi Merlin.  Adding RC gate filtering has another benefit: This acts as soft start and in this case charging the output capacitor will be slow and 10ohm resistor will be useless. Also adding high impedance resistor in the RC filter will decrease the capacitors value. 10uF@400V is about 2 times larger than 4.7uF. Depends by the ripple voltage this capacitor can be lower: 1uF or even 470nF.

Niki