INTRODUCTION
The old Power Supply shown in the photo was based on the MC1466 floating regulator IC which has been discontinued in the mid-nineties. A new regulation circuit using two UA723 along with a Switching Power Supply Module have been fitted after the bulky and heavy Transformer had been removed. May this Blog inspire you to build, modify or improve your own Power Supply.
Features
- Linear Floating Regulator (suitable also for Output Voltages >40V)
- Output Voltage Adjustable to Zero Volts
- Short Circuit Protection
- Constant Current & Constant Voltage Indicator
- Voltage/Current Regulation with Automatic Crossover
- Over Voltage Protection in Case of Intermittent Potentiometer Wiper
- 2x UA723 Design for Precise V & I Regulation
Theory of Operation
The Main Supply is an AC/DC converter with 24V/3.2A output rating adjusted to 28V. Powered by the main supply a DC/DC converter provides the auxiliary regulated 15V and moreover isolation for the floating operation of the regulator circuit. The auxiliary 15V supply is floating with respect to power ground. Its ground is ca. 6.8V below VO.
There are two similar circuits, one for Voltage Control and one for Current Control, comprised of an adjustment potentiometer, current source and error amplifier with output VC (pin 11 on UA723).
A constant current is fed into the adjustment potentiometer, which in turn, develop a constant reference voltage for the error amplifier.
The connected collectors VC of both error amplifiers allow only one mode of operation. Depending on the mode of operation one error amplifier output VC controls the base current into the output stage. This Darlington Output Stage drives the load current by amplifying the base current with a DC current gain > 1000.
Two op-amp comparators provide a low active output signal for Constant Voltage and Constant Current mode.
For safety reasons there is no mains switch. But there is switch for the auxiliary supply to turn off the power at the output terminals.
Noise performance is poor, caused by the AC/DC and DC/DC converters.
Error Amplifier
In order for the Error Amplifier in the UA723 to function properly the following conditions must be met:
- Error Amplifier (pin4 & pin5) Common Mode Input Range VCM:
2V < VCM < (VREF + 0.7V) - Output (pin10): VO > VCM
- V+ (pin12) > 9.5V
Voltage Control
U5 produces a current 1.2V/R7 though RV2. The voltage developed across RV2 is then compared with the output voltage by the error amplifier U6. In order to minimize temperature drift, RV2 and R7 have a temperature coefficient TC < 100ppm and the current through both resistors is kept low.
U5, a 3-Terminal Linear Regulator, operates with an output current as low as 0.3 mA. Unlike the LM317L which demands at least 5 mA.
In the event of a short circuit of the output stage or a voltage is applied to the output terminals, R8 limits the current through the potentiometer RV2. The rated current for a 10k multiturn potentiometer is about 15 mA. D2 and D3 protect U5 and U6 when charging/discharging C4 rapidly.
In the event of a wiper failure of RV2 the current source U5 ramps up the voltage across C4 and thus VO to VIN which may damage sensitive loads. U7C prevents this hazard. During normal operation pin 3 and 2 of RV2 are at the same potential (not loading pin 3). During a failure pin 3 is pulled to Power Ground, buffered by U7C pin 8 which in turn limits the output voltage VO to about 1V.
Current Control
U3 produces a current 1.2V/R5 though RV1. The voltage developed across RV1 is then compared with the voltage across the shunt resistor R4 by the error amplifier U4. In order to minimize temperature drift, RV1, R4 and R5 have a temperature coefficient TC < 100ppm and the current through resistors RV1 and R5 is kept low.
Series and Parallel Connection of Power Supplies
Caution when connecting supplies with different output voltage/current ratings.
Parallel Connection
General:
The supply with the higher voltage rating may damage the other one depending on the voltage settings. In most cases there is no protection against Over Voltage.
The supply illustrated here:
Do not apply a voltage > 30V to the output terminals. A Voltage > 30V may damage the AC/DC Converter RS-75-24.
Series Connection
General:
The supply with the higher current rating may damage the other one depending on the current limit settings. A Reverse Voltage situation may occur on the supply with the lower current rating.
In most cases a diode across the output terminals is the protection against Reverse Voltage.
The supply illustrated here:
Do not draw a current > 3A from the output.
Reverse Voltage Protection
General:
When using accumulators or batteries with a power supply always put a Polyfuse in series with the leads. Make sure that the fuse trips within a few seconds during a Reverse Voltage event.
The supply illustrated here:
A 60V/3A Schottky diode is the protection against Reverse Voltage.
Output Stage Consederations
A heatsink assembly for one TO-3 transistor has been chosen to fit the limited space on the back panel. As a rule of thumb a non-destructive Power Dissipation in the range of 20W to 50W (Natural Convection) is achievable.
Heat sink: Fischer SK 08 75 for TO-3 case, R_sink_ambient=1.8K/W (Natural Convection)
TO-3 case: R_junction_case=1.5 K/W (BDX65, 2N3055, etc.)
TO-3 insulator: R_case_sink= 1 K/W
R is denoting the thermal resistance here (unit K/W or °C/W).
T_ambient =20 °C (for lab use)
T_junction =??? (max. 200°C for BDX65)
Since the above parameters are fixed, the junction temperature will rise with higher power dissipation. Its temperature rise per watt depends on the total thermal resistance of the heatsink assembly and transistor used. For a real-life temperature measurement the power supply was adjusted to dissipate Pd=30W. Measurements were taken after the thermal settling.
Theoretically expected junction temperature (Pd=30W):
T_junction= T_ambient+Pd*(R_sink_ambient+R_case_sink+R_junction_case)
= 20°C+30W *(1.8+1+1.5)[°C/W]
T_junction= 149°C
This calculated result leaves a decent margin of 51°C for T_junction.
Real life (Pd=30W):
The junction temperature is derived from the temperature of the TO-3 case.
T_sink= 65°C
T_case= 100°C
T_junction= T_case+Pd* R_junction_case = 100°C+ 30W*1.5K/W
T_junction= 145°C
Dissipating the full output power of Pd=84W will fry the TO-3 transistor:
T_junction= 20°C+84W *(1.8+1+1.5)[°C/W]= 380°C ( max. 200°C for BDX65)
In real life the transistor will survive for several seconds due to thermal inertia if the assembly is at ambient temperature.
Practical Limitations
- Permanent RS-75-24 operation should not exceed the rated 75W (28V/ <2.7A)
- Permanent output current for a save one-transistor operation:
IO=1A+2A*VO/25V
Power dissipation limitations in general can be reduced or overcome by:
- Forced Convection (fan)
- Paralleling Pass Element Transitors (see Reference List, Section 4)
- Current Foldback
- Tap switching of the power transformer’s secondary winding
Output Stage Modifications
The SOA (Safe Operating Area) of the output/pass transistor(s) limits the usable collector current significantly at high collector-emitter voltages. A series connection of pass transistors will increase the collector current margin.
Modes of Operation
This power supply is designed to operate as a constant voltage source or as a constant current source. Automatic crossover to either mode of operation occurs when load conditions change as follows:
Constant Voltage
The power supply will function as a constant voltage source while the load current does not equal the current value, ILIM, set by the CURRENT LIMIT ADJUST. When load current VO/RLOAD = ILIM the supply will cross over automatically and will operate as a constant current source. Further decrease in value of load resistance RLOAD, results in decrease of voltage across the load Vo while its current remains regulated to ILIM.
Constant Current (Automatic Crossover)
The power supply will function as a constant current source while the load voltage Vo does not equal the voltage value set by the VOLTAGE ADJUST. When load voltage Vo, equals the value set by the VOLTAGE ADJUST, the supply will automatically cross over and operate as a constant voltage source.
Power Supply Hardware
Line Transient Response (Power Up, Down)
Load Transient Response
Reference List
MOTOROLA LINEAR/SWITCHMODE VOLTAGE REGULATOR HANDBOOK
Section 4: Series Pass-Element Considerations for Linear Regulators
Page 30: Floating Regulator Configurations
Page 59: Heatsink Calculation
Page 55: Semiconductor Mounting Considerations
Page 235: MC1466L, MC1566L Voltage and Current Regulators
Page 249: MC1723, Positive or Negative Adjustable Regulator