Hi Wouter,

At 16.27 2015.02.28, Wouter van Ooijen wrote:
>Electron schreef op 28-Feb-15 om 11:10 AM:
>> Hello,
>> please take a look at the following circuit:
>> http://oi57.tinypic.com/256h0fa.jpg
>>
>> I use a high voltage (500V) PNP transistor as a high side switch.
>> With a PIC pin I turn it ON or OFF, through the NPN high voltage=20
>(500V) transistor.
>> The 200K resistor is to partially turn ON the transistor when the=20
>PIC is in tri-state (e.g. during brief reset time). Then the PIC will=20
>be either ON or OFF, so it will be able to withstand also the leakage=20
>from that 200K resistor (2 mA worst case, the protection diodes can=20
>withstand it). 200K and 2K resistors are of course rated for 400V=20
>(actually I use a couple of 200V equal resistors (half resistance) in seri=
e).
>>
>> Basicly the PIC knows the voltage of both sides, and turns ON the=20
>switch when the left side voltage is higher than the right side=20
>voltage, with some exceptions.
>> It's like a diode, but being in software it allows me to do things=20
>that a diode cannot do.
>> Switching speed can be in the KHz region. The impedance is not=20
>really low (currents are 100 mA peak max, and limited by the source anyway=
).
>>
>> Now the question: do you think that the Rb resistor is really necessary?
>> If so, why?
>>
>
>Static case: Without that resistor the off-current (leakage) of the NPN=20
>will be amplified by the worst case (in this case: highest) Hfe of the=20
>PNP. The resulting current will flow through the PNP, even when it is=20
>supposed to be off. The Rb is there to 'take' this current without=20
>causing the BE junction of the PNP to conduct. So this resistor must be=20
>less than 0.6V/(NPN leakage).
>
>Dynamic case: When the NPN switches off, the charge on the base of the=20
>PNP will keep it conduction for some time. The Rb resistor will amek it=20
>switch off faster.
>
>Suggestion: add a 5V zener from NPN base to ground, and put the 2k in=20
>the emitter lead. That makes the NPN stage a constant current sink, so=20
>you will have more predictable properties.

But will it still be possible to turn it ON and OFF via the (3.3V) PIC?

With kind regards,
Mario


>
>Wouter van Ooijen
>
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