# Simple HV power supply with ripple problems

#### Sjöholm

Joined Jun 13, 2018
35
Hejsan all,

I'm building a simple HV power supply, but no matter what smoothing cap I throw at it, the ripple is mostly unchanged - I guess I need a special capacitor, but which?

I tried MKP and foil capacitors ranging from 1nF to 200nF to no avail.

This is the LTSpice sim. I'm using CL01-12 diodes (didn't have the spice model).

And this is how it looks on the scope (multiply voltage with 1000):

#### crutschow

Joined Mar 14, 2008
34,697
So how much ripple can you tolerate?

#### Sjöholm

Joined Jun 13, 2018
35
So how much ripple can you tolerate?
So that I can put a IRG4PC50W across the output and switch it, I tried with the ripple as is, but the darn thing wouldn't turn on.

Also I'm a bit baffled by the fact that even with a 1nF cap the sim shows a few volts ripple, with a 900nF foil cap in real life I can't get even close to what the sim shows.

#### wayneh

Joined Sep 9, 2010
17,498
So that I can put a IRG4PC50W across the output and switch it, I tried with the ripple as is, but the darn thing wouldn't turn on.

Also I'm a bit baffled by the fact that even with a 1nF cap the sim shows a few volts ripple, with a 900nF foil cap in real life I can't get even close to what the sim shows.
Do you really want R1 to be 1G? That's a fantasy in my world. What happens in the sim if you take that down to 1M or less?

#### Sjöholm

Joined Jun 13, 2018
35
Do you really want R1 to be 1G? That's a fantasy in my world. What happens in the sim if you take that down to 1M or less?
I use a 1G glass resistor in series with a 1M film resistor (not in the diagram) to make a voltage divider so I can measure various HV's. The 1G will obviously not be the final load.

As per your suggestion I tried with various values of R1 in the sim, and only at values below 4K you start to see some ripple about 10V, the current being around 200mA which the transformer can supply.

So with 1G I have about 200V ripple, why? I'm perplexed.

#### sghioto

Joined Dec 31, 2017
5,421
So with 1G I have about 200V ripple, why? I'm perplexed.
What is the DC voltage output?
SG

#### Sjöholm

Joined Jun 13, 2018
35
What is the DC voltage output?
SG
As you can see on the scope picture, it is 600V peak with 200V ripple.

#### sghioto

Joined Dec 31, 2017
5,421
Those CL01-12 diodes are for microwave power supplies. Will they operate at 50KHz?
SG

#### thedoc8

Joined Nov 28, 2012
162
As you can see on the scope picture, it is 600V peak with 200V ripple.

I see 200mv ripple not 200volt and what do you expect to filter with 100n. What am I missing.

#### Sjöholm

Joined Jun 13, 2018
35
I see 200mv ripple not 200volt and what do you expect to filter with 100n. What am I missing.
You missed "And this is how it looks on the scope (multiply voltage with 1000)" i.e. 200mV * 1000 = 200V

#### Sjöholm

Joined Jun 13, 2018
35
Those CL01-12 diodes are for microwave power supplies. Will they operate at 50KHz?
SG
Yes, and I tried with some 2CL77 Fast Recovery, the ripple problem persists.

#### Sjöholm

Joined Jun 13, 2018
35

#### wayneh

Joined Sep 9, 2010
17,498
Can you experiment with turning down the frequency? And goosing C1 up to 1µF or more.

#### Sjöholm

Joined Jun 13, 2018
35
Can you experiment with turning down the frequency? And goosing C1 up to 1µF or more.
It's a fixed zero voltage crossing transformer exciter so I can't adjust freq.

But I think I have figured it out now, apparently the ~600nA though the 1G resistor wasn't enough to turn on the diodes properly, so I switched that to a 1M resistor and it now looks like below. I will keep the 1M resistor as ballast. Case closed. Thank you all for your suggestions.

#### wayneh

Joined Sep 9, 2010
17,498
Do you really want R1 to be 1G? That's a fantasy in my world. What happens in the sim if you take that down to 1M or less?
Suspected as much.