e01b012.PDF

POWER SUPPLY

Power Supply (1)

Part 1:

analogue electronics controlled by microcontroller

Every electronics laboratory needs a powerful regulated bench supply. In

the model described here a microcontroller monitors the actual and

desired current and voltage settings.

There are many types of laboratory

benchtop power supplies available,

from the very simple to the highly

sophisticated. They range from lin-

ear, purely analogue supplies with

only voltage regulation to switching

supplies with both current and volt-

age regulation using a microcon-

troller, also offering programmable

signal patterns and various monitor-

ing functions, while also driving an

IEEE interface.

Our benchtop power supply lies

in the middle of this spectrum.

Depending on the rating, it offers a

voltage range of 0-25 V at up to 2.5 A

or 0-20 V at up to 1 A. The voltage

and current settings are adjustable

digitally using four buttons each

(two for coarse and two for ﬁne con-

trol). These provide a resolution of

100 mV in voltage and 10 mA in cur-

rent. The desired and the actual volt-

age and current values are shown on

a backlit LCD panel. The device can

be controlled remotely over its built-

in RS232 interface, and the measured

current and voltage values are avail-

able continuously over this interface.

The communication protocol uses

only ASCII characters, so that, in the

simplest case, HyperTerminal suf-

fices to display and adjust the val-

ues. A proper power supply control

Elektor Electronics

11/2001

Digital Benchtop

POWER SUPPLY

program is also available for free

download from the author’s website

at www.pic-basic.de . The source

code of this program is also pub-

lished, so that it can be modiﬁed for

other applications.

A Microchip PIC16F84 microcon-

troller controls the power supply,

running a program written in PIC-

BASIC 1.3. The source code, assem-

bler listing and machine code hex

dump for this program are also freely

available from the address above.

DIGITAL PSU SPECIFICATION IN BRIEF

2,5-A version

1-A version

Input voltage 230V

–0/+10 % @ 2.5 A / 25 V

10 % @ 2.5 A / 24 V

10% @ 1 A / 20 V

Set voltage accuracy

+30 mV typical

Set current limit accuracy

+5 mA typical

Output voltage ripple

5 mV (voltage regulation)

10 mV (current regulation)

5 mV (voltage regulation)

25 mV (current regulation)

Classical

analogue regulation

The circuit consists of an analogue

part and a digital part, which are

assembled on the same circuit

board. Only the pass transistors, the

LCD module and the mains trans-

former (along with the mains input

and switch) are not fitted to the

board. The classical analogue regu-

lator circuit is brought up-to-date

using an LT1491. This IC is a quad

operational amplifier, which has the same

pinout and practically the same (or better)

characteristics as the legendary LM324. Also,

the LT1491 offers rail-to-rail operation on its

inputs and outputs. Table 1 shows a few of

the characteristics of the two ICs. The LT1491

earns its place in this circuit despite its

R45

TR1

Ω

Fl1

T2, T3, T4 = TIP142

0W5

R46

1N4148

Ω

0W5

C19

IC1

(1k8)

R47

24V

80VA

63V

Ω

2A T

4A T

0W6

(21V)

B80C5000

(B80C1500)

(2A T)

C1'

0W5

C12

10n

+5V

557B

63V

P600D

(1N4007)

10 000

R16

10k

50V

T5, T6, T7 = BC547B

R43

(4 700

35V)

R17

39k

R18

0...25V

0...2A5

R15

10k

+5V

IC5

SRG8

IC1.B

R19

8k2

10k

1N4148

+5V

IC7

C1/

IC1.D

78L05

oder

Drahtbrücke)

Ω

R20

100n

R14

10k

IC1.C

150

C20

IC1.A

Ω

R21

IC2

16V

R13

IC1 = LT1491

R10

R22

18k

12V

1W3

63V

R23

Ω

100n

(39k)

+5V

R12

R11

R24 ...R33 = 1%

74HC164

R34

Tant

+5V

R40

R41

IC4

IC5

IC6

IC4.D

+5V

IC4.A 13

C18

R42

100n

IC4 = 4066

IC4.C

IC4.B

+5V

GND

+5V

MCLR

+Ub

IC6

SRG8

C16

IC3

RB7

+5V

16V

RB6

RA0

R38

100k

R44

C1/

RB5

RA1

R/W

V+

PIC16F84

IC2 = TLC272

+5V

C13

C1+

RB4

RA2

RA3

C10

TXD

RB3

R37

100k

IC8

R35

CTS

16V

4n7

C1–

RB2

RA4

R39

IC2.B

T2OUT

T2IN

RB1

IC2.A

RXD

R2IN

R2OUT

RB0

2k5

R1IN

R1OUT

OSC1

OSC2

T1OUT

T1IN

C15

LCD

1x 16

C14

C2+

500

Ω

MAX232A

16V

R36

16V

C2–

C11

74HC164

V-

C17

4MHz

100n

16V

000166 - 11

Figure 1. Circuit diagram of the 2.5 A power supply. Values in brackets refer to the 1 A version.

11/2001

Elektor Electronics

470

POWER SUPPLY

higher price. With the aid of a few close-tol-

erance metal ﬁlm resistors the op-amps work

sufficiently accurately that only a few cali-

bration points are required.

In a power supply with classical analogue

regulation such as the circuit in Figure 1 , the

output voltage and current are measured and

continuously compared against the desired

values by comparators. For this, the output

voltage is taken directly from the output con-

nectors (thereby avoiding errors due to the

resistance of the tracks and contacts) and

brought into the input voltage range of the

comparator using a simple voltage divider.

Current measurement is somewhat more

complicated, since in general a shunt resistor

is required in the current feedback loop. The

resistance of the shunt must be as low as

possible in order not to reduce the output

voltage range too severely and to keep the

power losses low, but on the other hand must

be high enough and accurate enough to

ensure that the voltage drop (which is pro-

portional to the current) is practically mea-

surable by the comparator and not lost in the

offset voltage or noise of the electronics.

The shunt resistance consists of ten 1 Ω

resistors (R24-R33) wired in parallel. This is

rather cheaper than a 0.1 Ω resistor with a

tolerance of 1 %, and the expected deviation

from the true value is rather lower. The layout

is organised so that only a little track resis-

tance affects the measurement.

With an output current, or load, of 2.5 A,

0.25 V is dropped across the shunt resistance.

Op-amp IC1.C multiplies this voltage by a

factor of 20, in the 2.5 A version, and 40, in

the 1 A model. This ampliﬁed voltage is mea-

sured and taken to the comparator via R14.

However, the current also has an effect

on the voltage measurement. Op-amp IC1.B

divides the voltage drop across the shunt by

four and inverts it, giving –0.0625 V. If the

voltage across the output terminals (i.e.,

across the load) is 25 V, then 25.25 V relative

to the circuit’s ground appears at the posi-

tive output terminal. Voltage divider R7/R8

and R15 have a total value of 50 k

Table 1. LM324 vs LT1491

LT1491

(typical values)

LM324

(typical values)

Offset voltage

200...350 µV

2 mV

Offset voltage drift

2 µV / °C

7 µV / °C

Input current

4 nA

45 nA

Power supply voltage (max.)

44 V

32 V

Short-circuit output current

25 mA

20 mA

would affect the measurement of

the output voltage. So that the op-

amps can operate with negative

voltages relative to ground a nega-

tive auxiliary supply is required,

which is derived from the input

voltage with the aid of diode D1.

The divided-down, ampliﬁed and

compensated measurement volt-

ages are finally taken to the invert-

ing inputs of op-amps IC1.A and

IC1.D via R9 and R14. Op-amp

IC1.D, connected as a comparator,

compares the actual voltage (at the

inverting input) with the desired

voltage, and IC1.A does the same

for the current. The desired values

originate from the PIC16F84 micro-

controller (IC3) which can generate

two precise analogue voltages

thanks to R/C combinations R11/C9

and R12/C8. The voltages are pro-

duced using pulse-width modula-

tion of the microcontroller’s output

pins. These two D/A converters

have a resolution of 8 bits.

C6 and C7 cause the two com-

parators to exhibit a lowpass

response. The outputs drive the base

connection of the pass transistors

via D5 and D6 in a ‘wired OR’ conﬁg-

uration. If one of the measured val-

ues exceeds the desired value, the

base is pulled towards ground (or

even slightly below) and the transis-

tors are switched off. If neither the

desired current nor the desired volt-

age is reached, constant current

source T1 delivers a base current of

2 mA (for a 2.5 A power supply) or

360

ter resistors are provided. If the volt-

age across one of the three resistors

rises above 0.65 V (at around 1.3 A),

one of the transistors T5, T6 and T7

will turn on and thereby turn off the

base current to the Darlington tran-

sistors. This affords effective protec-

tion against current spikes that may

occur if the output is short-circuited

and which would otherwise damage

the power transistors. In the 1 A

power supply only one Darlington

transistor is ﬁtted. The emitter resis-

tors and protection circuit can there-

fore be dispensed with. For the 2.5 A

power supply a 0.6 K/W heatsink is

required, and for the 1 A power sup-

ply a 2.4 K/W heatsink is fitted. At

maximum load — that is with a

short-circuited output at the highest

current possible — a temperature of

80 °C will be reached. The two ver-

sions of the power supply are there-

fore (without active cooling) suitable

for continuous use at full load.

. Across

the voltage divider we have

25.25 V+0.0625 V=25.3125 V. Of this, 4/5th is

dropped across R7 and R8, and 1/5th is

dropped across R15. Across R15 we there-

fore have a drop of 5.0625 V. At the voltage

divider output (at the junction of R8 and

R15) we therefore have exactly 5 V relative

to the circuit’s ground. The voltage at the

output of the voltage divider (relative to

ground) therefore reflects exactly the output

voltage, independent of the current flowing

in the shunt resistor. Without this compen-

sation for the voltage error the voltage

across the shunt resistor (up to 0.25 V)

Ω

Values set

by the microcontroller

The connecting link between the

analogue and digital domains is the

PIC microcontroller IC3. In order to

accurately measure and produce

voltages an accurate and stable ref-

erence voltage is of course required,

which it derives from its supply volt-

age. For this reason a few compo-

nents are added around D2 (pre-reg-

ulation to 12 V) and IC7, a ﬁxed volt-

age regulator. The pre-regulated

+12 V supply is also used to feed

IC2.

Using R3 and P1 the voltage at

the regulator output can be set to

5.12 V (in practice 5.14 - 5.16 V). R3

provides a basic load so that a cur-

rent of at least 33 mA always flows

through P3. This current is chosen to

be about 10 times as great as the

A (for a 1 A power supply),

independent of the output voltage.

Such a low base current is only

practical because power Darlington

transistors (T2, T3 and.T4) are used.

For the 2.5 A unit three Darlington

transistors are fitted. In order that

the current is properly shared

between the transistors, 0.51

Ω

emit-

Elektor Electronics

11/2001

POWER SUPPLY

current in the ground connection of

the voltage regulator. Load-depen-

dent variations in the common-pin

current therefore only have a very

small effect. Moreover, the load cur-

rent varies proportionally less owing

to the high basic load current.

As a result of these measures, the

reference voltage is very stable and

changes only as a consequence of

the normal ageing of the compo-

nents. It is therefore recommended,

as with any other electronic test

equipment, to recalibrate the device

after a few years.

The reference voltage reference

voltage is also used for the delta-

sigma A/D converter built around

IC2. Using this conversion technique

a high-precision converter can be

built with little circuitry, and, inde-

pendent of component tolerances,

very accurate measurements can be

achieved with good reproducibility.

However, 1 % resistors have been

used to simplify offset calibration

using P2. The microcontroller also

plays a part, of course, in the ana-

logue-to-digital conversion. Conver-

sion proceeds in the following

stages:

In the initial state, microcontroller

pin A2 (set as an input) is high

impedance and pin A3 (an output) is

at 0 V. The voltage to be measured is

present at the non-inverting input of

IC2.B, and, since the op-amp is con-

nected as a buffer, also at its output.

Let us suppose that this voltage is

exactly 1.28 V. Then the output of the

integrator IC2.A will be at the level

of the positive supply. Pin A2 on the

microcontroller will interpret this as

a high logic level. When a measure-

ment is to be carried out, the micro-

controller changes the state of pin

A3 from low to high (i.e. to 5.12 V)

and waits until the voltage on A2

changes to logic low. Now, by tog-

gling pin A3 the microcontroller

holds the output of the integrator in

the middle of its range. The varying

output signal of the integrator will

be interpreted by the microcontroller

as high or low. The mark-space ratio

of the integrator’s output is now

measured. It can be seen that pin A3

must be held high for three times as

long as it is held low: this is so that,

when combined together with the

voltage being measured, a voltage

can be produced at the inverting

input to the integrator which on

average is exactly the same as the

voltage on its non-inverting input,

namely 2.56 V.

The offset with P2 is required,

because this type of A/D converter

does not operate with negative volt-

ages: the microcontroller’s algorithm

would fail. As well as allowing for

compensation for component toler-

ances, P2 also provides a small posi-

tive offset. A TLC272, with MOS

inputs, is selected for IC2 because

the A/D converter will only work

accurately when the input currents

are considerably smaller than those

achieved with bipolar ICs. This ben-

efit is bought at the cost of a rela-

tively high offset voltage. Here how-

ever, where an adjustment must be

made anyway, this is no real disad-

vantage.

IC4 is an analogue switch that

connects either the voltage at pin 11

(for voltage measurement) or at pin

8 (current measurement) to buffer

IC1.B.

loaded into the register by the microcontroller

using pins B5 (data) and B6 (clock). In the

case of IC6 these values are control com-

mands and character codes which are trans-

ferred to the LCD module, running in 8-bit

mode, using a strobe signal on pin B7. In the

case of IC5 these form a bit pattern which

allow the microcontroller to identify (via pin

B4) when a button has been pressed. P3 is

the potentiometer required for adjustment of

display contrast. The wiper being at the

ground end corresponds to a viewing angle

of between 10° and 20° above vertical: this is

the recommended setting when the unit is to

be used on the bench.

Microcontroller pins B0, B2 and B3 com-

prise an RS232 port. Using the well-known

MAX232 the signal levels are shifted

between ±10 V on the RS232 side and TTL

levels on the microcontroller side. In addition

to the required connections (RXD and TXD),

the CTS (clear to send) signal is also con-

nected. The RS232 interface offers remote dis-

play as well as remote control. This will not

concern us further here: a description of the

software and operation of the unit will be pre-

sented in the second part of this series, in the

next issue. Let us turn instead to the con-

struction of the power supply.

Interfaces

The microcontroller has several inter-

faces at its disposal. The keyboard

interface is implemented using IC5

and the LCD panel is connected via

IC6. These are 8-bit shift registers

(type 74HC164) with serial input and

parallel outputs. Digital values are

(000166-1)

The construction of the Digital Benchtop PSU will

be described in next month’s issue.

11/2001

Elektor Electronics

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