Typical regulator current boost transistor. - Page 1
Forum section: electronics / projects

author: davelectronic
author_rank: Frequent Contributor
timestamp: September 02, 2023, 11:53:09 am
content: I'm looking for a linear regulator current boost transistor, but just a single transistor that can handle 8 Amps to 10 Amps of current. The circuit is the typical linear voltage regulator with a single PNP pass transistor. I know HFE drops with increased current, and data sheets show the HFE current gain, with a voltage of about 5 Volts, and the HFE drop as the current increases. I know 5 Amps is possible from a single pass transistor, but is there, say a Darlington transistor that is available that is PNP and can carry up to 10 Amps at 12 volts with good heatsinking. I have looked at some high power PNP Darlington transistors, but not sure if they can handle 10 Amps at 12 volts. Your thoughts and suggestions appreciated.

author: Ian.M
author_rank: Super Contributor
timestamp: September 02, 2023, 12:07:44 pm
content: Why not use a discreteSziklai pair, so you can use a power NPN pass transistor with a lower power and more readily available PNP driver?It halves Vbe (vs. a Darlington) and has much lower Vce_sat, so the boosted regulator will need less 'headroom'.

author: davelectronic
author_rank: Frequent Contributor
timestamp: September 02, 2023, 12:28:27 pm
content: Thanks for replying, is that the circuit where the NPN transistor drives a PNP transistor, if that's the right way round.

author: davelectronic
author_rank: Frequent Contributor
timestamp: September 02, 2023, 12:54:57 pm
content: So I would use the Sziklai pair in the PNP configuration with a 12 volt positive voltage regulator ? But is that Sziklai pair made up of Darlington transistors, or is it just PNP and NPN transistors. Something like the TIP2955 transistor and the TIP3055 transistor, to supply the 10 Amps I am looking for at 12 volts.

author: Ian.M
author_rank: Super Contributor
timestamp: September 02, 2023, 01:13:49 pm
content: The Szlakai driver needs to be an ordinary BJT, not a Darlington, otherwise you loose the advantage of lower Vbe. However the output transistor can be a Darlington if the regulator's min headroom (dropout voltage - nom. output voltage) is sufficient to accommodate its increased Vce_sat.A TIP3055 may not be suitable for 10A load current due to its SOA limits. Assume 9A through it, and the maximum voltage drop it can withstand (with active cooling) is only 9.5V. In practice it isn't cost effective to use such cooling vs multiple pass transistors, or a transistor with greater Ic and power ratings.

author: macboy
author_rank: Super Contributor
timestamp: September 02, 2023, 01:16:41 pm
content: You said 10 A at 12 V, but you are unclear whether this means 12 V output (probably) or a 12 V drop on the transistor. It is the voltage drop, not the output voltage, that determines the power dissipated in the pass transistor.The TIP2955 is only a 90 W device, and that assumes you can keep the case very cool (unrealistic with a passive heatsink). You also need to consider the SOA of the transistor, even if the power looks ok. This is usually a bigger consideration at higher voltage and lower current due to second breakdown. Also consider that base current x Vbe counts toward the power dissipated in the transistor. A single 2955 is probably not going to suit your application, but higher power PNP exist.

author: Ian.M
author_rank: Super Contributor
timestamp: September 02, 2023, 01:24:38 pm
content: OTOH in a Szlakai configuration, the PNP driver only has to handle the NPN output transistor's base current, which will be an order of magnitude lower so its much easier to find a suitable driver transistor, and for the output transistor, high current high power NPNs are much more readily available than equivalent PNPs. Using multiple output transistors (with individual emitter resistors for current sharing) with a single driver is also an option, and may well be cheaper and easier to heatsink.

author: davelectronic
author_rank: Frequent Contributor
timestamp: September 02, 2023, 01:29:35 pm
content: Yes I get it I think, the base voltage drop is to high if the first transistor in the Sziklai pair is a Darlington transistor. But the second transistor can be a Darlington transistor. The overall output voltage would be 12 volts. I have seen Sziklai pair configurations with a resistor between the pair for faster switching or pull down, would I need to consider this resistor between the Sziklai pair. I also see the SOA an issue at 10 Amps for the transistors I have mentioned like the TIP2955 and the TIP3055 transistors. I could find two other transistors with higher maximum power ratings. But if I had enough voltage head room, is there any benefit if both transistors in the Sziklai pair are Darlington transistors.

author: davelectronic
author_rank: Frequent Contributor
timestamp: September 02, 2023, 01:38:40 pm
content: I have built power supplys with multiple pass transistors, from two transistors, and up to six transistors. They work great for high current, I've drawn up to 20 Amps at 12 Volts from the six pass transistors. I just wanted to see if it's possible to use a single transistor, or Darlington transistor, or this Sziklai pair to obtain up to 10 Amps 12 volts output. Most of the time I would be needing 8 Amps at 12 volts with a 50% duty cycle. The 10 Amps at 12 volts is just a theoretical maximum output, a bit of power in reserve. Thanks again for the help.

author: Ian.M
author_rank: Super Contributor
timestamp: September 02, 2023, 01:43:52 pm
content: If the Sziklai gain is too high you'll run into stability issues and may even end up with a RF power oscillator, so using a Darlington driver is usually not worth the trouble. Even with only a Darlington output transistor, you'll probably have to roll-off the driver's HF gain with a small capacitor or series RC negative feedback network connected C-B to prevent oscillation. You may even need such a network for a simple (non-Darlington) Sziklai pair to improve the transient response to step load changes.

author: BrokenYugo
author_rank: Super Contributor
timestamp: September 02, 2023, 02:00:42 pm
content: If you want to get that much power through a single power transistor I'd suggest a buck regulator.

author: davelectronic
author_rank: Frequent Contributor
timestamp: September 02, 2023, 05:23:00 pm
content: I will look up this filter network incase I do need it. I've had some minor issues in the past with the TIP147 where the voltage was almost 2 volts higher than the expected 12 volts. I think that might have had ocillation problems. I gave up with that, as the voltage just wouldn't come down to the regulator voltage.

author: davelectronic
author_rank: Frequent Contributor
timestamp: September 02, 2023, 05:28:10 pm
content: I've used buck and boost regulators many times in the past, I wanted linear power as opposed to switching power. Buck and boost converters are really good, I use a high current boost converter to power my mains drill on a drill stand press, that gives me variable drilling speeds nicely.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: September 02, 2023, 08:10:32 pm
content: Quote from: davelectronic on September 02, 2023, 11:53:09 am120W from a single package is the biggest challenge here, and SOA is also tricky.Darlington also have poor dropout performance.Even with active tunnel cooling, some spread of the thermal load ( more packages) makes this practical.I would suggest multiple decent (HFE > 150 ) PNP parts, which can use base resistors for current sharing.If you really want one device, digikey have 2N5684G that might do 10A in TO3, with fold back current limiting.

author: davelectronic
author_rank: Frequent Contributor
timestamp: September 02, 2023, 09:03:45 pm
content: Yes I thought it might be difficult from a single transistor. Thanks for the suggestion, I have been digging around, as I knew I had a couple of TO3 high power Darlington transistors, the MJ11033G it's a 300 watt transistor. Unfortunately it is a Darlington, I don't know how that would fair as a single pass transistor. A while ago I did try a pair of MJ11015 Darlington transistors, and they worked well, I got 12 Amps at 12 volts from those, but they where on a substantial heatsinks. I just wanted to see if I could get 8 Amps to 10 Amps at 12 volts from a single transistor. That current load would be used at a 50% duty cycle, so not flat out continuous.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: September 02, 2023, 09:22:31 pm
content: Quote from: davelectronic on September 02, 2023, 09:03:45 pmIt will work, it just needs more voltage headroom.Since you have some, why not try them ?

author: davelectronic
author_rank: Frequent Contributor
timestamp: September 02, 2023, 09:34:15 pm
content: Yes I will give the MJ11033G a go, nothing to lose. I did look up the other high powered transistor you suggested 2N5684 if I've remembered. I've got two 300VA toroidal transformers 115V x2 and 18 -0 volts x2 windings. But one should be ok for 10 Amps. And a couple of 300VA 12-0 x2 windings, but was saving them for a bigger project, if I can get 12 volts regulated out of one of them (another ongoing project) but I will try the MJ11033G first, if it fails, I will buy a couple of the 2N5684 high power transistors.