ST AN2446 APPLICATION NOTE

AN2446

Application note

STEVAL-IHT002V1

Intelligent thermostat for compressor based on ST7Ultralite MCU

Introduction

The STEVAL-IHT002V1 is a very low-cost evaluation board designed with the intent to
replace the existing mechanical thermostat.

While driving a compressor, the evaluation board is able to drive the RUN winding, replace
the PTC and drive directly the START winding too. Stall rotor detection is also implemented.
Both functions are oriented to reduce the total power consumption.

The evaluation board is based on the new low-cost, 8-pin, 8-bit ST7Ultralite microcontroller
(MCU), which controls the entire process. The MCU is equipped with a programmable Flash
memory, 1 MHz internal clock source and runs 1 KByte C-based software.

Even if the evaluation board is especially designed for driving small-size or mid-size
compressors, it is fully customizable and adaptable to any other application where
thermostat or temperature control is required.

An STMicroelectronics Patent Application is pending for the compressor control device and
the method for controlling a compressor described in this document.

This document provides a complete description on how to customize the evaluation board.

STEVAL-IHT002V1 evaluation board

February 2007 Rev 1 1/13

www.st.com

Contents AN2446

Contents

1 STEVAL-IHT002V1 overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2 Circuit description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

3 How the system works . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3.1 Cabinet temperature regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3.2 Dead-time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3.3 Compressor start-up sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3.4 Stall rotor detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

4 Scope waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

5 Customization: software and hardware modifications . . . . . . . . . . . . . 9

5.1 Cabinet temperature offset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

5.2 START winding duration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

5.3 Stall rotor control duration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

5.4 Dead-time duration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

5.5 Gate pulse duration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

5.6 Disabling the stall rotor detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

5.7 Different START winding management . . . . . . . . . . . . . . . . . . . . . . . . . . 10

6 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

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AN2446 STEVAL-IHT002V1 overview

1 STEVAL-IHT002V1 overview

The STEVAL-IHT002V1 evaluation board includes a capacitive power supply on the left side
with the AC switches located in the middle of the board. Both AC switches can work without
a heatsink. The temperature sensor used to detect the stall rotor condition is mounted on
the top of the AC switch driving the RUN winding.

The 8-pin MCU and the ICC programming connector are on the right side of the board.

Figure 1. STEVAL-IHT002V1 evaluation board

Figure 2. STEVAL-IHT002V1 (reverse angle)

On the left side are the potentiometer to set the cabinet temperature and the connector
where to plug the external NTC sensor. The connector where to plug the compressor and
the voltage mains is on the right side. The AC switch on the left is the one driving the START
winding and the right one drives the RUN winding.

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Circuit description AN2446

2 Circuit description

The circuit can be divided into a few simple parts.

VDD (5 VDC) is obtained by means of a classic capacitive power supply directly taken from
the mains (220 VAC 50 Hz). Capacitor C1 is sized to provide about 30 mA. VDD supplies
the MCU and all the analog circuitry.

Potentiometer P1 sets the temperature of the cabinet. The signal is sent directly to the MCU
for A/D processing (pin 2). The cabinet temperature is read by means of a negative
temperature coefficient (NTC) thermistor. This signal is also sent directly to the MCU for A/D
processing (pin 3). MCU also generates the gate pulses to drive the AC switches. The gate
current is about 15mA for each AC switch.

Since pin 4 is not able to deliver a current higher than 5mA, a transistor is necessary. Pin 6
indeed has a high current capability up to 20mA; therefore the sole gate resistor is enough
to drive the AC switch.

The gate pulses have to be synchronized with the mains voltage: pin 7 captures the signal
and generates an external interrupt when the mains voltage crosses the zero, on the falling
edge only (one time per period). A resistor divider is applied to pin 5 of the MCU. R13 is
glued on the top of the AC switch driving the RUN winding. R13 is a temperature sensor
used to detect the stall rotor condition by reading the temperature of the AC switch. The
signal is then processed directly by the MCU. The evaluation board is equipped with an ICC
connector to in-circuit program the MCU (care has to be taken while programming the MCU,
please refer to the User's manual for more details).

Figure 3. STEVAL-IHT002V1 schematic

U1

+VDD

+VDD

R14
10K

1%

P1
50K

R2
12K

1%

R3
5K

R4
12K

1%

NEUTRAL

LINE

J1
CON3

MAINS

+VDD

Neutral

C6
47nF

275~ X2

Line

ST7ULTRALITE

1

VDD

2

PA5/AIN4/ CLKIN/ei4

3

PA4/AIN3/MC0/ei3

4

PA3/RESET

R6
10K

C4
10nF

C1
1uF

275~ X2

ei0/ICC DATA/ATPWM/AIN0/PA0

+VDD

R7
10K

Q1
BC337

R1

1W

47

ei1/IC CCLK/AI N1/PA1

ei2/LTIC/ AIN2/PA2

D1
1N4007

D2
1N4007

VSS

C3
100nF

1
2
3

+VDD

+VDD

12
34
56
78
910

TR1
ACST67S

J3
CON3

START

3

RUN

2

LINE

1

+VDD

L1

6.8uH

R5
10K

J2
CON10A

ICC Programmer

R9

COM

300

G

OUT

*

R10

3W

47

8

7

6

5

C5
1nF

DZ1
5V6

1/2W

R11
560K

+VDD

R12
100K 1%

R8
300

C2

+

1000uF

16V
105°C

+VDD

R13
100K

+VDD

COM

G

TR2
ACST67S

OUT

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