Agilent E3610A circuit questions - Page 1
Forum section: electronics / projects

author: Burning Circuits
author_rank: Contributor
timestamp: May 04, 2020, 11:39:57 am
content: Hi,this is my first topic on EEVblogI have a question related to the circuit of the Agilent E3610A power supply.Someone can explain to me the function of Q4 and Q5 BJT?Another question: there are pages said that MOSFETs are not so good in a linear application, what do you think about Agilent choice?

author: pqass
author_rank: Super Contributor
timestamp: May 04, 2020, 03:20:12 pm
content: Quote from: Burning Circuits on May 04, 2020, 11:39:57 amI would guess safety.If a voltage source (eg. battery) back fed from the output terminals is higher than the power-supply set voltage then the bases of Q4 and Q5 would be higher than their emitters, thus shutting down the main MOSFETS. Unless its so much higher than the filter capacitor voltage at C2 (forward biasing through the MOSFET body diodes) that I'm not sure what would happen.Also, since there are two transistors, it's more likely that Q4 and Q5 aid in balancing current through each MOSFET pair (Q1+Q6 vs Q3+Q7). If one pair is hotter, then its Rds_on is higher, which produces a higher voltage drop across the pair reducing its current output contribution. The other MOSFET pair's output is contributing more current thus has a higher output voltage (at its source terminal, WRT ground) than the other pair at the same point. Whichever base of Q4 or Q5 is higher than it's emitter starts to turn on, thus lowering the gate of the higher current contributing MOSFET. Normally, I've seen you just need to rely on R35 and R36 series resistors which are typically < 1ohm to do balancing.

author: xavier60
author_rank: Super Contributor
timestamp: May 04, 2020, 04:33:24 pm
content: With my bench supply project, I found that allowing the series pass MOSFET to be driven with more than 6V didn't cause any useful drop in on resistance and caused too much extra delay in turning off the MOSFET after drive saturation, a likely cause being during regulation dropout.Maybe the B-E junctions of Q4 and Q5 are being used as Zeners. I can think of no reason for the C-B junctions being forward biased with current from -12 via R43.

author: Kevin.D
author_rank: Frequent Contributor
timestamp: May 04, 2020, 05:25:06 pm
content: They are a diff amp (long tail pair) to ensure MOSFETS share the load equally. the bjt's are shown connected in reverseactive mode here but they would still operate to some extent (with a lower gain). We came to the conclusion that this was a typo on the schematic with some schematics showing correct orientation according to another poster.Here's a link to an earlier discussionhttps://www.eevblog.com/forum/projects/agilent-3611a-powersupply-question/msg406874/#msg406874.Regards

author: Kleinstein
author_rank: Super Contributor
timestamp: May 04, 2020, 06:13:57 pm
content: Not all MOSFETs are suitable for linear operation. However at a relatively low voltage the old type used by HP are OK for linear operation. It is especially the newer (e.g. past 1995) MOSFETs made for lower voltage switching that don't like linear operation.The direct parallel connection of 2 MOSFETs each is a little odd. One would normally avoid this.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: May 06, 2020, 01:21:03 am
content: Hi group,I just happen to have an open E3610A on my bench today. So here are a few pictures:I bought this at a Ham Radio Flea market. It didn't work. I found out it is missing the mains transformer.These are picture of Q1 and Q3.The power transistors are not MOSFETs. They are NPN Darlington transistors. Transistors Q4 and Q5 are not fitted.Schematic from the manualAll the parts marked with a * you have to look up on the parts list. This reveals the following:The E3610A and E3611A both use two 2N5065 Darlington transistors and emitter resistors for sharing.The E3612A uses MOSFETs and the long-tailed pair Q4 and Q5 to force sharing. Q4 and Q5 are NPN they are listed as 2N2222A on the parts list.Regards,Jay_Diddy_B

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: May 06, 2020, 01:37:38 am
content: Hi,I also have the E3612A in my collection. Here are picture of the output sections, confirming MOSFETs and Q4, Q5:Regards,Jay_Diddy_B

author: Burning Circuits
author_rank: Contributor
timestamp: May 06, 2020, 09:51:50 am
content: I'm designing a bench power supply (0-30V, 0-5A), based on the attached circuit with various improvements.I was interested in MOSFET power stage for better load balancing ... but seems that these introduce other problems.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 06, 2020, 08:33:20 pm
content: A NPN darlington can have a preformance relatively similar to a MOSFET. So the same / similar circuit could work with both options.The MOSFETs have the advantage of less control current which may help with the precision of the current reading. They can also be faster, but they miss the kind of inherent peak current limiting. Load balancing is also more difficult with FETs and thus the extra transistors.

author: opa627bm
author_rank: Regular Contributor
timestamp: August 01, 2022, 06:01:33 am
content: I been trying to understand the operation of this powersupply too, and from the board layout looks like Q4 and Q5 are marked backwards.however, if I ran simulation, looks like the circuit will still work with VBE= 6V+will that break the BE junction ?