# How to design a HV start-up circuit

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#### refugee

##### Member level 1
I'd like to design a HV start-up circuit with zero static current. Does it possible? Somebody told me that it's an impossible task.

#### erikl

##### Super Moderator
Staff member
I guess the fact that you don't get an answer would show that nobody understands your problem!? At least I don't. High Voltage? 5V? 1kV? Start-up what? A current mirror? An oscillator? A company? You better provide a little bit more info! ;-)

#### Prototyp_V1.0

An ideal inductor will work this way.

But it will also cause an DC source to short after a short while.

#### WimRFP

erikl said:
I guess the fact that you don't get an answer would show that nobody understands your problem!? At least I don't. High Voltage? 5V? 1kV? Start-up what? A current mirror? An oscillator? A company? You better provide a little bit more info! ;-)

Hello erikl: I have same problem, the posting doesn't provide sufficient info to give useful reply.

#### refugee

##### Member level 1
erikl said:
I guess the fact that you don't get an answer would show that nobody understands your problem!? At least I don't. High Voltage? 5V? 1kV? Start-up what? A current mirror? An oscillator? A company? You better provide a little bit more info! ;-)

Please forgive me I did not make it clear.
Let's say that I need to design a boost converter. Vin=5~30V. So, I need design an internal power supply block. In this block, I designed a constant gm to generate current. So, I need a start up circuit.
But I don't know how to design a start up circuit with zero static current. The attachment is my circuit.
The only way to reduce the static current is to increase the resisitor. If R=2M, Vin=20V, the static current is 10uA. It's too large.

#### erikl

##### Super Moderator
Staff member
refugee said:
... The only way to reduce the static current is to increase the resisitor. If R=2M, Vin=20V, the static current is 10uA. It's too large.
Why not use a capacitor instead of the resistor? It's a short circuit at start, then loads up, and finally the current tends to zero. After switching off, you'd have to wait for a while before a new start.

#### WimRFP

Hello,

When this circuit is for start up only, can you disconnect the resistor from ground after the main process runs?

Other method is to store some energy in a capacitor and use an external pulse or thyristor/diac like circuit to generate the current pulse required for starting the main process.

The thyristor like approach works independently of input dV/dt as you need a minimum stored charge before the thyristor fires. I use this approach in power oscillators to start them.

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