transistor rating question - Page 1
Forum section: electronics / projects

author: 84750Erik
author_rank: Contributor
timestamp: March 15, 2025, 05:14:45 pm
content: In the schematic attached there is transistor Q5 (C1815 SOT-23, spec sheet attached) that in theory should withstand the conditions to which it is subject:Vcb max 60V, Vce max 50V, Ic max 150mA, Pc max 200mW.The circuit runs fine in prototype modus (with 1x 1000µF 63V filter condensor, not the pack of 4x 4700µF).Yet when connecting even one 4700µF condensor, after several hours of test run and on/off switchings, the circuit becomes unstable with output voltage not being properly regulated anymore.This even resulted in one burnout of Q5.Q5 serves as the driver for the regulated output voltage, so my guess is that it must have good Hfe and frequency characteristics.On power up the Vbe junction of Q5 is subject to a very fast -1V pulse. On power-down Q5 (Vbe) is subject to a -3V (attached pictures below, probe set at x10, so multiply voltages x10).After the blow-up i replaced Q5 with a TIP41C (TO-220), and the circuit functions good, ie good output voltage regulation. But the TIP41C is quite a power transistor with probably not the best frequency characteristics.Questions:1. What might have cause dthe blow-up of Q5?2. is it relevant/advisable to replace Q5 (SOT23) with a bigger transistor?3. Would a BD139 be a good substitute?4. ...or other alternatives/solutions?

author: Swake
author_rank: Frequent Contributor
timestamp: March 17, 2025, 07:34:24 am
content: Hi Erik,Quote from: 84750Erik on March 15, 2025, 05:14:45 pmThis seems to suggest that the type of Q5 is not the culprit, unless it was pushed beyond it's limits by design but that does not seem to be the case as at first sight any general low power transistor would fit for Q5.Quote from: 84750Erik on March 15, 2025, 05:14:45 pmGood find. A transistor does not like negative Vbe (actually it is the negative current flowing that kills it). Sounds like this is the cause of Q5 failing.Quote from: 84750Erik on March 15, 2025, 05:14:45 pmBecause of it being much sturdier than the 2SC1815 it is also much more tolerant to a negative Vbe. Yet, it still does not like to be exposed to a negative Vbe...I would suggest putting a switching diode in parallel (1N4148, cathode at base of Q5, anode at emitter) to protect the transistor.

author: 84750Erik
author_rank: Contributor
timestamp: March 17, 2025, 08:05:42 am
content: Great analysis, many thanks for taking the time @Swake . I will include that diode in the schematic and test it out and report back (but that will be in a month's time, we are abroad on holiday for a month since yesterday).

author: Swake
author_rank: Frequent Contributor
timestamp: March 17, 2025, 08:16:46 am
content: Even better would be to avoid the negative Vbe at all. But I'm not all too familiar with the 723 beyond the power supply published in the 80'ies in Elektor.

author: 84750Erik
author_rank: Contributor
timestamp: March 17, 2025, 08:27:07 am
content: Quote from: Swake on March 17, 2025, 07:34:24 amWhat is your take on Q1 package: currently an SOT23 (200mW), would TO92 (400mW) not be better, or is the SOT23 adequate?

author: 84750Erik
author_rank: Contributor
timestamp: March 17, 2025, 08:31:36 am
content: Quote from: Swake on March 17, 2025, 08:16:46 amThis project has nostalgic value: I had a working LM723-based supply since my college years in the 80's which I decided to upgrade to a proper pcb. The nice thing about this design is that the output voltage can be regulated to 0,0V.I never had issues with the original PSU, on which the current schematic is fully based; but I may be overlooking something that causes this issue.

author: magic
author_rank: Super Contributor
timestamp: March 17, 2025, 08:52:55 am
content: The whole design is odd and troublesome.As far as I see, the internal opamp drives the internal power transistor whose emitter is grounded through the internal zener. Then its collector current passes through Q5 and controls Q2 and Q3/Q4.There is no limit to the current through the internal zener, power transistor and Q5. Even if you upgrade Q5, you may endanger the chip internals. I would suggest disconnecting Vz from ground and connecting a resistor from OUT to ground which would allow no more than a few mA with OUT voltage at maximum. The chip's VCC appears to be regulated to ~17V, so this should provide reliable protection.editSuch change would reduce loop gain, but you probably aren't suffering from lack of that.Alternatively, maybe a resistor between Vc and Q5 emitter would work, with almost no effect on loop gain.Not so great, it still fails to protect the internal driver transistor. I dunno if it's protected enough by its limited base current.I also suspect that R8 is unnecessary.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: March 17, 2025, 08:53:07 am
content: Quote from: 84750Erik on March 17, 2025, 08:27:07 amThe failure may not be reverse VBE at all. The swing is negative, but not by a huge amount.More likely is excess base current.The 18V rail is emitter follower, so is very stiff, and the 723 collector output can sink significant current with no series limiting R.The 1k limits the Q5 collector current, but it is of no help to limit base-emitter current.A power part, is of course more rugged on all current ratings.

author: 84750Erik
author_rank: Contributor
timestamp: March 17, 2025, 04:17:40 pm
content: Quote from: PCB.Wiz on March 17, 2025, 08:53:07 amTo remedy the Q5 base current I am inclined to insert a 470R resistor between pin 11 and emitter of Q5.I noticed this was the case in the very original schematic, for some reason it had been omitted. Time to get it back?

author: magic
author_rank: Super Contributor
timestamp: March 17, 2025, 05:13:08 pm
content: It will protect Q5 and the internal power transistor, but still allow "unlimited" current from the internal driver transistor through the power transistor's BE junction into the internal zener.It would be safer to limit power transistor's current by disconnecting the zener from ground and using an external resistor instead.

author: 84750Erik
author_rank: Contributor
timestamp: March 17, 2025, 06:43:35 pm
content: Quote from: magic on March 17, 2025, 05:13:08 pm...and would that still allow the output voltage to be driven to 0,0V?

author: magic
author_rank: Super Contributor
timestamp: March 17, 2025, 08:50:46 pm
content: Hmm, good question. 723 internal output stage can't go much lower than error amplifier's common mode voltage.If there are issues, the resistor could be added in series with the internal zener, to move output stage operating point back to above 6V. I suppose 1kΩ between the Vz pin and ground would work and allow ~10mA max through Q5.