POWER SUPPLY DESIGN
when required. This ensures circuit complexity is kept to a minimum.
Summary
Thanks to modern technology losses can even be reduced by up to 84 percent in a basic circuit that has been tried and tested over the years and used millions of times. The microcontroller used keeps the additional expenditure and costs low.
The circuit presented can be used for “Power Factor Correction” applications as well as all other applications. From an economic perspective the circuit is suitable for approximately 2 A per diode and above. The microcontroller can be used for other tasks, such as monitoring the load current, the mains voltage, or the rectified voltage. This information can be processed on the secondary side and displayed if necessary via a galvanically isolated interface (for example, I2C).
Figure 3
bridge rectifier for “Power Factor Correction,” which is also suitable for all other applications.
Requirements
The same switching behaviour as that of a diode rectifier must be ensured for the implementation of a synchronous power rectifier. 1. The energy only flows in one direction, namely from the network to the load. 2. The switch (or diode) only conducts if the voltage and the current via the switch (or diode) are positive at the same time.
3. The switch should not conduct under any
other circumstances. Figure 3 is a block diagram of the implemented circuit that fulfills the specified requirements. The load current is measured with the
resistance Rshunt, filtered with an operational amplifier, and scaled to the AD converter input of the MSP430.
The phase and neutral are connected directly to a digital input via a high-impedance resistor network (R1, R2, R3, R4). The protective diodes integrated in the MSP430 limit the input voltage to the supply voltage or ground. This is a very simple, but effective method of retrieving logic signals with a higher voltage than the supply voltage of the microcontroller. The load current Iload and the mains voltage
are sampled every 25 us. A digital filter and a plausibility check are utilized in order to avoid malfunctions.
The voltage, which represents the load
current, is compared to a fixed threshold value. This is selected in such a way that ensures a certain load current flows through the diodes. If the MOSFETs are not switched on, a higher threshold value is used than when the MOSFETs are active. A hysteresis is achieved as a result and the system becomes more immune to malfunctions.
The status of the three elements is stored in one variable each. If the digital input is a logic one (phase, neutral), the variable is incremented by one; if it is a logic zero, it is decremented by one.
It is implemented for the load current analogous to the voltage measurement. If the converted value is higher than the threshold value, a variable is incremented by one, otherwise it is decremented by one. The variables are limited to a minimum of 0 and a
Table
maximum of 3 by the software.
If a variable reaches the value 3, a logic one state has been ascertained three times in a row. A variable value of 0 corresponds to logic zero state on three consecutive occasions. Only if these three variables are 0 or 3, they are checked for plausibility and, where appropriate, the MOSFETs are activated. This ensures that a stable state is established. Malfunctions are also filtered out by this method. There are only two status combinations in which two MOSFETs may be switched on (see table 3). This ensures that the current flows from the mains to the load and that the voltage between the phase and neutral is applied correctly to the MOSFETs. The four MOSFETs must be switched off for all other combinations. Figure 4 illustrates the program sequence. MOSFETs Q1 and Q4 (lo-side) can be controlled directly by a digital output, as the ground connection of the MSP430 is connected to the source. A somewhat higher expenditure is necessary for Q2 and Q3, as their source is connected to the phase or neutral. Gate drive transformers are used in order to achieve galvanic isolation. The square wave signal (200 kHz) required for this is generated directly by the timer module of the MSP430 and it is switched on or off
Texas Instruments |
www.ti.com
Milan Marjanovic & Matthias Ulmann are power design engineers at Texas Instruments
Figure 4: Flow diagram
14 CIE Power Supplement April 2012
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