ST AN1953 Application note

AN1953

Application note

PFC for ST7MC starter kit

Introduction

The aim of this Application Note is to give an example of how to use the ST7MC microcontroller to implement the Power Factor Correction (PFC) inside a motor control application. The ST7MC microcontroller is provided with a Motor Control Peripheral which has been developed to implement motor control function for Induction motors and Permanent Magnet Synchronous motors. The motor control peripheral leaves the microcontroller resources free to perform the power factor correction.

The power factor correction technique utilized is called “transition mode”. The hardware used to realize the system is the ST7MC Starter Kit plus an additional external power board called “Add On”.

In this application note, theoretical information about the PFC control method is given, in addition to an explanation of the software routine able to manage the PFC stage and the calculation of the Add On components.

The reader can find extra information about the motor control related routines the application note AN1904.

AN1953 Rev 2 1/20

http://www.st.com

PFC for ST7MC Starter Kit

Contents

1 Theory about the PFC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.1 Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2 Starter kit approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3 Firmware description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3.1 Flowchart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3.2 PFC software description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

4 Hardware description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

4.1 Power stage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

4.1.1 Power section design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

5 Schematics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

6 Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

7 References and related materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

8 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

2/20

PFC for ST7MC Starter Kit 1 Theory about the PFC

1 Theory about the PFC

This application note focuses on the management by a microcontroller of the boost stage to obtain a power factor corrector (PFC) working in transition mode.

Of course to do this job the microcontroller should have a timer with a special feature.

A block diagram of the application is shown in Figure 1.

Figure 1. Block diagram

1.1 Description

With this method, the boost inductor works on the boundary between continuous and discontinuous mode. In this operation mode there is a high peak current which means that this kind of approach could be used for power below 600W.

Here, the system works with fixed ON-time and variable frequency and functions as follows:

The main supply is rectified by the bridge and the energy is stored in the transformer during the turn-on period of the mosfet. This time interval is called Ton and must be calculated by the Micro according to the Vout value, the input voltage and the load.

To simplify the algorithm, just the output voltage is measured and the difference (error) between the desired values (400V) and the actual value is calculated. This error is multiplied by a constant to obtain the right correction value.

BOOST

Stage

DRIVER

uC Supply

LOAD

ST7MC

Volt

Measure

The measured voltage value is the average of eight ADC conversions.

The turn-off time of the mosfet is called Toff and is fixed by the circuit. This means the inductor discharges all its own energy to the output stage until its current goes to zero.

In this condition, the drain Voltage will go to the Vin voltage and an oscillation due to the MOS capacitance is added to this voltage.

In order to reduce the commutation losses, the MOS must be turned ON when the Drain voltage reaches the minimum.

To detect this condition the secondary winding of the transformer is used. In fact, in this secondary winding is a square wave with zero mean value which is made up of a negative signal during the Ton and a positive signal during the Toff. To adapt this signal to the digital world of a microcontroller a resistor to limit the current plus the internal clamping diodes have been used to obtain a square TTL waveform.

3/20

1 Theory about the PFC PFC for ST7MC Starter Kit

So, finally, a TTL square wave is obtained on the microcontroller pin which is used by the timer peripheral to restart the counter cycle and fix the counter period.

For the timer, it's important to note that if there isn't a load variation and/or a variation of the input voltage, it could work as a standalone circuit without any computational time required to the micro. In fact, if the zero current condition (ZCD) is missed the timer will restart at the minimum frequency through a software routine.

Of course the availability of the micro makes a lot of features possible simply by modifying some parameters and it also allows a particular software routine to be used to improve the control method.

The output of the PWM timer is connected to the TD220 driver that drives the MOS of the boost stage.

4/20

PFC for ST7MC Starter Kit 2 Starter kit approach

2 Starter kit approach

To implement the PFC with the ST7MC starter kit we developed a power board “Add On” that realizes the control of the Bus voltage with a boost topology.

The Power Add On is connected to the mains and provides the Bus voltage for the starter kit. The Power Add On requires two connections with the starter kit. One is the PWM generated by the microcontroller coming from the starter kit and the second sends the zero crossing signals to the starter kit. The Add On power board needs a supplementary 15 Volts to supply the driver, which can be provided by the starter kit (J16). The starter kit is then connected to the motor by the wire for the phases and with the tachometer signal if required.

The Add On power board provides the rectified and controlled voltage bus for the starter kit. The starter kit board has been modified as in Figure 2

● the bridge D4 has been removed and correctly short-circuited (pin 1-2; pin 3-4)

● the capacitances C1 and C2 have been removed

● the NTC1 has been removed and short-circuited

● the fusible F1 has been removed and short-circuited

Figure 2. Modifications to be performed in the Starter Kit

J12

MOTOR

3 4 5

MSTBA2,54/5-G-5,08

1 2 3

GMKDS 3/3-7,62

90/140VAC

180/260VAC

TP13

Phase C

Phase B

STTA106U

NTC1

SG170(4R)

TP6

5X20(type KELSTONE)

F1 10A

R43

5.6K 1/4W

R49

5.6K 1/4W

R53

1K2 1/4W

D16

D17

R44

OR 1/4W

R45

R50

OR 1/4W

R51

R54

Mounted on 17°C/W AAVID Thermalloy heatsink

STBR608

C11

J11

MOTOR

PHASES

2 3 4 5 6 7

4.7nF

SOLDER FOR 110VAC

100nF 400V

BZW50-120

BZW50-100

TO SOLDER

The modification has been performed also on the value of some component in order to work in safe with a bus voltage of 400-500 Volt. So the following components have been substituted: (see Figure 3)

● C12 and C13 bulk capacitors have been replaced with one of 470uF 450Volt

● The transil Z2 and Z3 has been replaced with BZW50 -180

● R1 and R2 resistors have been replaced with 68K - 1W

● C20 capacitor has been replaced with 33nF - 600V

5/20

2 Starter kit approach PFC for ST7MC Starter Kit

Figure 3. Modifications to be performed in the Starter Kit

R20 6.2K

C13, C12: 470uF 450V

Z2, Z3: BZW50180 transil

C11

100nF

400V

BZW50-120

FOR 110VAC

BZW50-100

TO SOLD ER

C13

1000uF

200V

C12

1000uF

200V

R2 47K

R16

470K

HV Bus

R17 470K

R20

12K

47K 1W

TP11

C20:33nF 600V

Phase B

B60HDFP

7NB60HDFP

R21

0.047R

TP17

C20 33nF 400V

TP14

R1, R2: 68K - 1Watt

To get the Bus voltage value a voltage divider with R16-R17 and R20 is used. To get 5 Volts as a maximum voltage for the AD converter with the new Bus voltage max value, R20 must be replaced with a 6.2K resistance.

To connect the Power Add On board with the starter kit, use two control cables, one to communicate the PWM for the Add on coming from starter kit and one to get the zero crossing from the Add on to the starter kit. The connection of the zero crossing cable is critical. The shunt resistor has been connected near the pin of the starter kit to avoid a capacitive parasite effect of the cable acting as a filter for the zero crossing signal and a capacitor of 12pF has been added to filter the noise.

To generate the PWM for the PFC, the Timer B of the micro is used. The PWM signal of the PFC Power Add On is connected to the OCMP1_B pin of the micro (J10 Pin 3) and the Zero Crossing signal of the Add On reaches the ICAP1_B pin of the micro (J10 Pin 7).

The PE2 pin it is connected with ICAP1_B, because it is used to force the reset of PWM when the ZCD signal is missed.

This could happen for example when the instantaneous input voltage is too low to energize the inductor.

The Vout of the Add On is the Mains for the Starter Kit board so must be connected to the Pin 2 and Pin 3 of J3.

The ground signal is connected directly from the ground of the micro to the ground of the driver in the Add On.

The connections are depicted in Figure 4.

6/20

+ 14 hidden pages