HP8563E A6A2 regulator schematic-How does the circuit work? - Page 1
Forum section: electronics / projects

author: djsb
author_rank: Super Contributor
timestamp: February 21, 2022, 11:28:35 am
content: I have attached the schematic of the A6A2 regulator that is used in my HP8563E Spectrum Analyser. The positive regulator (+15v and +5v) share the same topology, only differing in the values of resistors (R205/R405 etc) and zener diode voltage.The negative regulators likewise share the same topology and have different component values.I am trying to understand how these regulators work. Could someone point me in the direction of an explanation. What is this component arrangement normally called? I know that the series pass MOSFET is controlled by the circuit and that the sense resistor is involved. I would just like to understand how the feedback mechanism works in each circuit. Thanks.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: February 21, 2022, 09:45:17 pm
content: Hi,Let me start with the +15V voltage regulator:The 5V reference enters on terminal E2. In normal operation CR201 is not conducting. There will be 5V on TP3. Voltage feedback is provided from terminal E4, via the voltage divider R208 and R209. Because the division ratio is 2:1 the voltage on E4 will three times the voltage on TP3. The amplifier U201A is a non-inverting amplifier. The MOSFET is a source follower.U201B provides over-current protection. In the event of an over-current Cr201 will conduct and reduce the voltage on TP3.Vr201 and R207, provide foldback, they reduce the current limit if there is a large voltage across the MOSFET. This reduces the MOSFET dissipation if the output is shorted.Regards,Jay_Diddy_B

author: djsb
author_rank: Super Contributor
timestamp: February 26, 2022, 06:40:55 am
content: Thank you for the explanation. I await the same for the -15v rails with anticipation. Thanks.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: February 26, 2022, 12:10:24 pm
content: Hi,The negative regulator:The voltage control loop. A -5V reference is applied to the inverting input of U301B via R302. Resistors R306 and R307 form a divide by 3 voltage divider. The output voltage is regulated by the op-amp U301B to 15V. The gain of the error amplifier is determined by C520, R302, and R308. Because the MOSFET is operated in common source, there is inversion. This why the reference is applied to the inverting input and the feedback is applied to the non-inverting input. In normal operation CR301 is not conducting.U301A provides over-current protection. R307 and V301 are used to reduce the current limit as the voltage across the MOSFET increases. This is done to reduce power dissipation in the MOSFET. When in current limit CR301 is conducting.Jay_Diddy_B

author: djsb
author_rank: Super Contributor
timestamp: February 26, 2022, 07:16:21 pm
content: The problem I have is with the -15v regulator. When the rail is loaded with 4.7 ohms, the current is 3.21 amps and the voltage is 15.06 volts (I'm using a Tenma 72-13210 DC load with the current sense connected to the TP303 test point). However, when I load the same rail with 4.41 ohms the current drops to 3.34 amps (should be 3.4 amps) and the voltage drops to 14.7 volts. The rest of the measurements are as followsLOAD EXPECTED CURRENT ACTUAL CURRENT VOLTAGE4.17R 3.6A 3.35A 13.973.95R 3.8A 3.36A 13.33.75R 4.0A 3.4A 12.6The +15v rail holds steady at +15v up to 4 amps. I have not tested the rails beyond 4 amps yet. I havent tested the 5v rails yet either.I have attached an LT SPICE FILE for the -15v supply. Likewise, I will upload one for the +15v rail and -5v rail in due course if needed. I'm hoping this will help me find out what may be wrong or if this behaviour is by design. Thanks.David.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: February 26, 2022, 08:04:33 pm
content: Hi,You have (at least one) mistake in the LTspice model:Jay_Diddy_B

author: djsb
author_rank: Super Contributor
timestamp: February 26, 2022, 09:27:02 pm
content: Corrected here. There maybe others but hopefully not.David.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: February 26, 2022, 10:52:28 pm
content: Hi,I found another error in the LTspice schematic, D2 anode was connected to the wrong place.I changed the load to a voltage controlled resistor.I added an output capacitor.The results are:I have attached the modified model.Jay_Diddy_BHP8563E_A6A2_MINUS_15_REG_JDB.asc(5.93 kB - downloaded 25 times.)

author: djsb
author_rank: Super Contributor
timestamp: February 27, 2022, 01:11:19 pm
content: I'm assuming that the power supply is working as it was designed to? I may check the 5v rails just for completeness. I'm going to put it back into the chassis and move on to checking the CRT and associated circuitry, and if I don't find a fault will move onto the line generator and blanking circuitry (the original problem is no CRT display). Thanks for your help.