Power Supply

[yngndrw]

I ment what would the regulation be like, I didn't think it told you in the datasheet ?

Edit: Just checked and it doesn't. Also how did you know that it's a 10A regulator example, in the datasheet it doesn't give any values for the examples ?

 

[audioguru]

The regulation of a regulator with a boost transistor won't be different because the regulator is working at much less than its max current rating so its regulation will be better.

The datasheet shows an old boost transistor with a max current of 7A. If you find a PNP transistor with a max current rating of 10A and if it has enough gain then it can be used instead. The series current-sensing resistor can have its value reduced so that the base current for the transistor is higher for low gain transistors.

 

[yngndrw]

The regulation of a regulator with a boost transistor won't be different because the regulator is working at much less than its max current rating so its regulation will be better.

The datasheet shows an old boost transistor with a max current of 7A. If you find a PNP transistor with a max current rating of 10A and if it has enough gain then it can be used instead. The series current-sensing resistor can have its value reduced so that the base current for the transistor is higher for low gain transistors.

Fair enough, thank you for explaining that.

If you was to use a darlington pair for it's higher gain, would I be right in assuming that the responce and regulation would be better (Due to the higher gain), however the regulator would have a higher dropout voltage ?

 

[audioguru]

A darlington transistor has a high current gain. Its voltage gain (or loss) is the same as any ordinary transistor so the response of the circuit won't be any different except darlington transistors are usually much slower than ordinary transistors.
Yes, a darlington transistor has a higher dropout voltage.

 

[yngndrw]

Ah I see, so it's voltage gain that's required.

Some of the examples have a PNP which then drives an NPN. Is this just because it's easier to find NPNs that have a higher current rating than PNPs ? Am I right in assuming that these will also suffer from a higher dropout voltage than the ones using a single PNP ? I guess this would be the reason why most low dropout voltage regulator ICs have a lower current rating than their "normal" dropout voltage counterparts.

Would a higher current gain have any benifits, such as less current lost through the resistors before the regulator IC it's self ? I guess that because more current would go through the transistor, it would theoretically allow an even higher over-all current rating for the circuit ? (As you're now limited by the rating of the transistor, not the regulator IC.)

Thank you for taking the time to walk me through this, I can be a bit slow sometimes. (Most of the time )

Edit: On a slightly different note, seems as we were talking about transistors.

Would an NPN setup with the Collector going to the load and the Emitter going to Ground (So placed after the load) be less efficient thank a PNP which is setup with the Emitter going to +VS and the Collector going to the load ? (So placed before the load)

Edit2: I've just looked at a couple of datasheets for high power transistors and it seems that they only have a gain of "Min: 20, Max: 70". I'm used to little transistors with gains of 200-500. Is this right or am I looking at shite transistors ? Am I right in assuming that with the correct base resistor and the correct resistor going into the Vin of the regulator (Current sense resistor ?) that these transistors would be fine ?

Would the max theoretical current through the circuit (Assuming the transistor and resistors could handle infinate current) be the current gain multiplied by the max current of the regulator IC it's self ?

Finally how are those two resistors before the regulator IC calculated and how is the PNP transistor dimensioned before the NPN ? Is the PNP calculated through the current rating of the NPN multiplied by the current gain of the NPN ?

Cheers.

 

[Hero999]

As a general rule, we assume the base current is 1/10 of the collector current if you want to drive the transistor on hard into saturation.

 

[yngndrw]

Ah I see, that will simplify working out the PNP's rating. Thanks.