Regarding this simple SMPS, is it possible to replace the MOSFET with a high-voltage BJT? Perhaps with some sacrifice in efficiency/power (which I am not too worried about)?

 

With some consideration, yes. You have to consider the recovery time from saturation. It is possible to drive gate without resistor however usually a small resistor will be used. Maybe not needed at low power levels.

 

Since the BJT is less efficient and thus will dissipate more heat, it may need a larger heat sink than the MOSFET.

 

Since the BJT is less efficient and thus will dissipate more heat, it may need a larger heat sink than the MOSFET.

Have you ever built a SMPS and then changed MOSFET to BJT?

 

With some consideration, yes. You have to consider the recovery time from saturation.

Interesting... I have a BUL216 that I was going to try.
It quotes a storage time of 3us, and fall time 600ns, with 200uH inductive load. Was do these times refer to, exactly?

http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00000009.pdf

 

For 3uS you get 333 Khz.

If your circuit is using maybe 40 Khz then don't care.

However the hFE is quite low, not sure if it can work well, I'd say no.

Think 12V 100mA -> 120V, 10mA.
12V 200mA -> 120V, 20mA.

Multiply with 0.7 maybe for losses.

You don't need such a big transistor (with low hFE).

 

Interesting... I have a BUL216 that I was going to try.
It quotes a storage time of 3us, and fall time 600ns, with 200uH inductive load. Was do these times refer to, exactly?

The storage time is the delay between the shut-off of the base current and the point at which the collector current starts to turn off. The fall time is the time it takes for the collector current to return to zero. Thus the total turn off time for the transistor would be 3.6μs. This, of course, limits that maximum switching frequency for the device. This is one of the reasons MOSFETs are often preferred for switching applications, since they have no storage time delay and also have a faster fall time.

 

A small transformer maybe would be better, since you don't need high voltage rating.

 

Thanks for the info everyone! I will see what other devices I can get cheaply. I'm only experimenting with the circuit, so it doesn't matter if I blow a few up.

 

Here you go:
http://uk.rs-online.com/web/c/?searchTerm=irfi7&sra=oss
They have the IRFI730 for £1.49/ea.

 

A small transformer maybe would be better, since you don't need high voltage rating.

The thread is discussing the transistor in a DC switching power supply with no transformer. I don't understand your comment. Did you post in the wrong thread?

 

The thread is discussing the transistor in a DC switching power supply with no transformer. I don't understand your comment. Did you post in the wrong thread?

Have you ever built any small SMPS?

1. There might be a heatsink or not.
2. They often use transformers.
3. There is no information that a transformer can not be used.

And, why not change it to have a transformer?

Did I post in the wrong thread?

May I ask you what you mean by larger heatsink?

 

I am talking about a SMALL transformer.

Many HV inverters use these, and not storage coils. But I don't suggest not to use a storage coil. It is only a possiblity. And it's unfair how crutschow is using this to suggest my reply is kind of useless or off topic. I don't accept that.

If I don't have enough knowledge about a thread topic, usually I don't write replies.

 

Have you ever built any small SMPS? I designed a 1A, 15V to 5V buck converter for a civilian space application. It used an inductor. Do you consider that small?

1. There might be a heatsink or not. True.
2. They often use transformers. True. And they often use inductors if they don't need isolation.
3. There is no information that a transformer can not be used. True.

And, why not change it to have a transformer? Why would you? The op's question was about substituting a BJT for a MOSFET in a circuit he already has. That seems to be only slightly related to whether an inductor or a transformer is used for the voltage conversion.

Did I post in the wrong thread? Only you can answer that.

May I ask you what you mean by larger heatsink? A heatsink that can dissipate more power for a given temperature rise.

My answers are above in blue.

I'm apologize if I thought your reply was off topic, but it wasn't clear to me why you mentioned using a transformer when the topic was about transistors.