Current version

r3B3b

Status

Completed, when used with toroidal main transformer and higher loads a forced cooling of enclosure’s interior is required

PCB manufactured

Yes (r3B3a)

PCB assembled

Yes (r3B3a)

BOM

Yes (Farnell, TME)

File repository

https://github.com/eez-open/psu-hw/releases/tag/1.0

(include Eagle, Gerber and BOM files)

License

TAPR v1.0

 

Fig. 1: Post-regulator module assembled (r3B3a)

 

The post-regulator module contains circuits for conditioning all input voltages required for the post-regulator module. It can be separated in two sections: “power” part (Fig. 2) and “biasing” part (Fig. 6)

Power pre-regulator

The power pre-regulator helps in keeping power dissipation of the post-regulator’s pass element within safe limits. That is accomplished by delivering output voltage that is only few volts higher then programmed level that appear at the post-regulator’s output. For example if output voltage is set to 10 V, the power pre-regulator output should be in average between 12 to 14 V depending of the load. Thanks to that total dissipation is limited to only few watts instead of tens of watts that will require much bigger heatsink or forced cooling that continuously works (and make additional unnecessary noise).

 

 

Fig. 2: Phase controlled mosfet pre-regulator

As power pre-regulator is used circuit presented here by EEVblog member blackdog. Mosfet Q2 is used as a switch to charge and keep capacitor’s C2 voltage few volts over programmed output. Tracking of output voltage is accomplish with Q1 and P1 can be used to set voltage difference. It’s also possible to replace it with Zener diode of e.g. 3 to 3.9 V. The difference has to be set in accordance with desired max. current. Smaller difference will further improve (decrease) power dissipation on the post-regulator side but if it’s too small a higher load current will cause post-regulator’s output ripple that goes below programmed output voltage level preventing post-regulator to deliver desired voltage.

Fig. 3 and Fig. 4 shows how pre-regulator (cyan trace) and post-regulator (yellow trace) outputs correlate with connected load of 1 A and 4.8 A. In later case is visible that pre-regulator is close to programmed output voltage and that is achieved experimentally by setting value of P1 to 240 Kohm. For model with max. output current of 3.12 A it can be set to ~ 80 Kohm (78.8 Kohm fixed value can be used) and that voltage difference remains preserved regardless of connected load.

Also note that bias voltage (magenta trace) stays almost unchanged and well above pre-regulator output voltage that is required for normal operation of N-Chanel mosfet used as pass element in the post-regulator.

 

 

voltage_difference_1A.png

Fig. 3: Voltage difference with 1 A load

An example of pre-regulator’s output ripple is shown on Fig. 5 (cyan trace) with corresponding Q3 gate control signal while delivering 3 A. Despite of it huge value (~ 2.5 V) it can be easily flatten out because of its low frequency.

voltage_difference_4.8A.png

Fig. 4: Voltage difference with 4.8 A load

 

Fig. 5: Pre-regulator output ripple (Vout = 50 V, Iout = 3 A, C3 = 560 pF)

Q2 switching time (Fig. 5 blue trace) is controlled by C3. The switching has to be synchronized with AC mains voltage that is provided using D1 and D2 that are connected to AC input from main transformer secondary coil.

Bias power supply

The bias power supply has two stages: high voltage SMPS pre-regulator and low voltage LDO’s for final regulation. The LM5574 (IC3) buck regulator from the TI Simple switcher family is in the core of the bias pre-regulator and has integrated power mosfet that is rated for output current of up to 500 mA while allowed input voltage could be up to +75 V. Principally it works like LM25575 on the auxiliary power supply with one exception – an coupled inductor TR1 is used to generate both positive and negative voltage. Pre-regulator feedback loop is very simple and derived from R22, R27 voltage divider that cannot guarantee good load regulation what in our case is really not important since final regulation is determined with LDO’s that follows. For that reason pre-regulator’s positive output is set to ~11.6 V what will generate at least -10.5 V on negative rail with connected load (post-regulator’s operational amplifiers).

IC1, IC2, and IC4 provide final regulated outputs of +5 V and -5 V for linear circuits and +5 V for digital circuits of the post-regulator module. The LP2951 is selected because it also provide “Power good” indication (ERROR output) that is used for checking power lines “health”. The MCU will not enable output when that signal is low.

 

Optional circuits

Fig. 6: Bias supply, sync isolator and temperature V/F converter

Two optional circuits are also added to the pre-regulator module: bias pre-regulator sync isolator and V/F (voltage-to-frequency) converter (IC6) used for channel’s related temperature measurement (e.g. of the output heatsink).

Switching frequency synchronization

LM5574 and LM25575 used on auxiliary power supply module have SYNC pin that can be used for switching frequency synchronization between multiple devices. Synchronization could be beneficial in suppressing undesirable sub-harmonic so-called beat frequencies and EMI effects. If multiple SMPS nodes shares same ground sync signal isolation is not required that is not our case. Therefore an single channel digital isolator (IC5) is used and two of them is enough to provide isolation between three devices. The SMPS controller with the highest programmed clock frequency will serve as the master and control the switching frequency of the all the devices with lower oscillator frequency.

One possible scenario is presented on Fig. 7. Connection between modules is accomplished with 3-wire cable (+5 V, SYNC and GND). One of the LM5574 on pre-regulator module is promoted to sync master and its RT or R24 value is set to 30 Kohm. Slaves’ frequency should not be too low otherwise erratic behavior is possible (not lower then 25 % of the frequency set on the sync master) therefore 33 or 36 Kohm could be used to set “free-running” frequency (without sync deployed).

 

Please note that SMPS controller (LM25575) on the Auxiliary power supply module cannot be master because it cannot provide isolated signal.

Fig. 7: Switching frequency synchronization

V/F converter for temperature measurement

Monitoring temperature of the heatsink on which regulation elements of both pre-regulator and post-regulator modules are mounted is of the paramount importance if the power supply has to continuously deliver high currents (2 A and above). For temperature measurement an NTC is used which resistance depends of surrounding temperature. That value is analog and if we want to monitor it digitally an A/D conversion device is required.

The simplest way to achieve that is to use some of MCU’s ADC inputs directly. More complex way is to use external ADC device as in case of measuring programmed voltage and current on the post-regulator module. That will require additional analog inputs or dedicated ADC device. Here added V/F converter is a simple way to “digitize” analog value by generating signal which frequency vary with NTC value. Resulting frequency is then possible to measure with the MCU. Such solution offers flexibility because it is not dependent of a MCU type used (if it has ADC inputs or not) and also do not require additional analog input on dedicated ADC.

Voltage (that is set by NTC’s resistance that depends of temperature) to frequency conversion is achieved using popular 555 timer device. It’s RC constant is defined by C26 and external 10 Kohm NTC and set to generate ~100 Hz per degree Celsius for temperatures over 25 oC. Output signal is delivered to one of the digital input of the I/O Expander located on the post-regulator module.

PCB layout

The PCB is designed to withstand up to 50 V and up to 5 A. It also provides place for an additional switching mosfet (Q1) if one wants to remove some load from the Q2. Please note that such configuration is not tested. This PCB also allows two options for the main capacitor (C3) – single Ø35 mm or three Ø22 mm capacitors.

 

Fig. 8: PCB layout (both sides, top, bottom)

Possible improvements and further development

  • Replace bridge rectifier with circuit using ideal diode bridge controller such as LT4320. That should further improve efficiency especially for 5 A model when power dissipation on rectifier diodes could goes above 7 W.