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| Is there any variable DC-DC converter suitable for pre-regulation? |
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| bloguetronica:
Hi prasimix, Why so many resistors in that feedback circuitry? Any particular reason? Kind regards, Samuel Lourenço |
| prasimix:
There is just one voltage divider (R3 + R9) that define max. output voltage (here about 60 V) that is well over max. working range (43 V). Max. voltage can be set much lower but it's intentionally in my case much higher then Vin (i.e. 48 Vdc) to push controller into 100% duty cycle by disconnecting PNP tracker to bypass pre-regulator. That is not applicable in your case. You can eventually remove R9 or decrease its value. |
| bloguetronica:
Thanks! And what about R8 in series with R7, is it to set the voltage difference higher? Kind regards, Samuel Lourenço |
| prasimix:
Hm, it's not really in series with R7: R8 is connected on Q2's base and set it's working point. Try to change its value in simulation and measure current thru it and check how it affect pre-regulator's output voltage (i.e. offset will increase for lower R8 or decrease for higher value). You can also use this simulation to see how such kind of tracker cannot provide the same offset over the whole range, e.g. for Vout_sim=0 V offset is above 4.5 V while for Vout_sim=40 V offset drops below 2.5 V. Therefore you can expect higher dissipation on post-regulator's pass element for lower voltages. |
| bloguetronica:
Hi prasimix, Finally had some time to run some simulations. Without R9, the circuit behaves linearly with the offset pretty much constant. I've then adapted the circuit to my needs (also changed the PNP transistor to the one I'm using). I'll not use the same DC-DC converter, but I hope the behavior is the same. The offset is set to around 3.6V, which I think is the ideal compromise, as it give some headroom so the pass element can regulate. Anyway, thanks for your excellent help. It was very valuable. Kind regards, Samuel Lourenço |
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