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Application Note 140 Part 2 / 3 - Fundamentals of Switching Mode Power Supply

Author: Henry J. Zhang, ADI company

Why switch mode power supply?

Obviously efficient. In SMPS, transistors operate in switching mode and nonlinear mode. This means that when the transistor turns on and conducts current, the voltage drop on the power path is minimal. When the transistor turns off and blocks high voltage, there is little current in the power path. Therefore, the semiconductor transistor is like an ideal switch. The power loss in the transistor can be minimized. High efficiency, low power consumption and high power density (small size) are the main reasons for designers to use SMPS instead of linear regulator or LDO, especially in high current applications. For example, today's 12vin, 3.3vout switching mode synchronous step-down power supply can usually achieve more than 90% efficiency, while the efficiency of linear regulator is less than 27.5%. This means that the power loss or size is reduced by at least 8 times.

The most commonly used switching power supply - step-down converter

Figure 8. Buck converter operation mode and typical waveform

Where ton is the on time interval in the switching cycle ts. If the ratio of ton / TS is defined as duty cycle D, the output voltage Vo is:

When the values of filter inductance L and output capacitance Co are high enough, the output voltage Vo is a DC voltage with only 1mV ripple. In this case, for 12V input step-down power supply, conceptually, 27.5% duty cycle provides 3.3V output voltage.

In addition to the above average method, there is another way to derive the duty cycle formula. The ideal inductor cannot have DC voltage in steady state. Therefore, the volt second balance of the inductance must be maintained during the switching cycle. According to the inductance voltage waveform in Figure 8, volt second balance requires:

Therefore, VO = VIN â€¢ D (5)

Formula (5) is the same as formula (3). This volt second balance method can also be used in other DC / DC topologies to derive the relationship between duty cycle and VIN and VO.

Power loss in Buck Converter

DC conduction loss

When the ideal components (zero voltage drop and zero switching loss in the on state) are used, the efficiency of the ideal step-down converter is 100%. In fact, power consumption is always associated with each power element. There are two types of losses in SMPS: DC conduction loss and AC switching loss.

The conduction loss of step-down converter mainly comes from the voltage drop generated by transistor Q1, diode D1 and inductor L when conducting current. In order to simplify the discussion, the AC ripple of inductive current is ignored in the following conduction loss calculation. If the MOSFET is used as a power transistor, the conduction loss of the MOSFET is equal to io2 â€¢ RDS (on) â€¢ D, where RDS (on) is the on resistance of MOSFET Q1. The conducted power loss of the diode is equal to IO â€¢ VD â€¢ (1 â€“ d), where VD is the forward voltage drop of diode D1. The conduction loss of the inductor is equal to io2 â€¢ r DCR, where r DCR is the copper resistance of the inductor winding. Therefore, the conduction loss of the step-down converter is about:

For example, the 12V input, 3.3v/10amax output step-down power supply can use the following components: MOSFET RDS (on) = 10m Î©, inductance rdcr = 2m Î©, diode forward voltage VD = 0.5V. Therefore, the conduction loss under full load is:

If only conduction loss is considered, the converter efficiency is:

The above analysis shows that the power loss of freewheeling diode is 3.62w, which is much higher than the conduction loss of MOSFET Q1 and inductor L. In order to further improve efficiency, ADI recommends replacing diode D1 with MOSFET Q2, as shown in Figure 9. This converter is called synchronous buck converter. The gate of Q2 needs to signal complement the gate of Q1, that is, Q2 is only on when Q1 is off. The conduction loss of synchronous buck converter is:

Fig. 9. Synchronous buck converter and its transistor gate signal

If 10m Î© RDS (on) MOSFET is also used for Q2, the conduction loss and efficiency of synchronous buck converter are:

The above example shows that the synchronous buck converter is more efficient than the traditional buck converter, and is especially suitable for low output voltage applications with small duty cycle and long conduction time of diode D1.

AC switching loss

In addition to DC conduction losses, there are other AC / switching related power losses caused by the use of unsatisfactory power elements:

1. MOSFET switching loss. Real transistors take time to turn on or off. Therefore, there is voltage and current overlap during on and off transients, resulting in AC switching loss. Fig. 10 shows a typical switching waveform of MOSFET Q1 in a synchronous buck converter. The parasitic capacitance of the top FET Q1, the charge and discharge of the CGD and the charge QGD determine most of the Q1 switching time and related losses. In the synchronous buck converter, the switching loss of the bottom FET Q2 is very small, because Q2 is always turned on after the body diode is conducted and turned off before the body diode is conducted, and the voltage drop on the body diode is very low. However, the reverse recovery charge of the bulk diode of Q2 may also increase the switching loss of the top FET Q1 and produce switching voltage ringing and EMI noise. Equation (12) shows that the switching loss of control FET Q1 is directly proportional to the switching frequency FS of the converter. It is not easy to accurately calculate the energy loss eon and eoff of Q1. For details, please refer to the application notes of MOSFET suppliers.

Figure 10. Typical switching waveform and loss of top FET Q1 in step-down converter

2. Inductance iron loss PSW_ COREã€‚ Real inductors also have AC losses related to switching frequency. Inductance AC loss mainly comes from core loss. In High Frequency SMPS, the core material may be iron powder core or ferrite. Generally speaking, the iron powder core is slightly saturated, but the iron loss is high, while the ferrite material is violently saturated, but the iron loss is low. Ferrite is a kind of ferromagnetic material similar to ceramics. Its crystal structure is composed of a mixture of iron oxide and manganese or zinc oxide. The main cause of iron loss is hysteresis loss. Core or inductance manufacturers usually provide power designers with iron loss data to estimate AC inductance loss.

3. Other AC related losses. Other AC related losses include gate driver loss PSW_ Gate (equal to vdrv â€¢ QG â€¢ FS) and dead time (when both top FET Q1 and bottom FET Q2 are off) body diode conduction loss (equal toï¼ˆ Î” TON Î” TOFFï¼‰ â€¢ VDï¼ˆQ2ï¼‰ â€¢ fSï¼‰ã€‚

In summary, switch related losses include:

Generally, it is not easy to calculate the switch related loss. The switching related loss is directly proportional to the switching frequency FS. In 12vin, 3.3vo/10amax synchronous step-down converter, the AC loss at 200kHz â€“ 500KHz switching frequency leads to about 2% to 5% efficiency loss. Therefore, the total efficiency under full load is about 93%, which is much better than LR or LDO power supply. It can reduce heat or size by nearly 10 times.

reference material

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