Introduction to calculation and prediction technology of loss of various components in switching power supply (SMPS)
A brief analysis of the main factors affecting the efficiency of DC-DC converters
This article introduces in detail the calculation and prediction technology of the loss of each component in the switching power supply (SMPS), and discusses the related technologies and characteristics to improve the efficiency of the switching regulator, so as to select the most appropriate chip to achieve high efficiency indicators. This article introduces the basic factors that affect the efficiency of switching power supplies and can be used as guidelines for new designs. We will start with a general introduction and then discuss losses in specific switching elements.
There must be energy consumption in the energy conversion system. Although 100% conversion efficiency cannot be obtained in practical applications, a high-quality power supply efficiency can reach a very high level, with an efficiency close to 95%.
The operating efficiency of most power ICs can be measured under specific operating conditions, and these parameters are given in the data sheet. Maxim's data sheet gives actual test data, and other manufacturers will also give actual measurement results, but we can only guarantee our own data. Figure 1 shows a circuit example of an SMPS buck converter. The conversion efficiency can reach 97% and can maintain high efficiency even at light load.
What's the secret to achieving such high efficiency? We'd better start by understanding the common problem of SMPS losses. Most of the losses in switching power supplies come from switching devices (MOSFETs and diodes), and a small amount of losses come from inductors and capacitors. However, if very cheap inductors and capacitors (with higher resistance) are used, the losses will increase significantly.
When selecting an IC, you need to consider the controller's architecture and internal components in order to achieve high efficiency metrics. For example, Figure 1 uses a variety of methods to reduce losses, including: synchronous rectification, integrating low on-resistance MOSFETs inside the chip, low quiescent current, and pulse-skipping control modes. We will discuss the benefits of these measures in this article.
The main function of a buck converter is to convert a higher DC input voltage into a lower DC output voltage. In order to meet this requirement, the MOSFET operates on and off at a fixed frequency (fS) under the control of a pulse width modulation signal (PWM). When the MOSFET is on, the input voltage charges the inductor and capacitor (L and COUT), through which energy is transferred to the load. During this period, the inductor current rises linearly, and the current loop is shown as loop 1 in Figure 2. When the MOSFET is open, the input voltage is disconnected from the inductor, and the inductor and output capacitor supply power to the load. The inductor current decreases linearly, and the current flows through the diode, and the current loop is shown as loop 2 in the figure.
The on-time of the MOSFET is defined as the duty cycle (D) of the PWM signal. D divides each switching cycle into two parts (D × tS) and ((1 - D) × tS), which correspond to the conduction time of the MOSFET (loop 1) and the conduction time of the diode (loop 2) respectively. . All SMPS topologies (buck, inverting, etc.) divide the switching cycle in this way to achieve voltage conversion. For buck conversion circuits, a larger duty cycle will transfer more energy to the load and the average output voltage increases. Conversely, when the duty cycle is low, the average output voltage also decreases. According to this relationship, the following conversion formula for buck-type SMPS under ideal conditions (without considering the voltage drop of the diode or MOSFET) can be obtained: VOUT = D×VIN IIN = D×IOUT. It should be noted that any SMPS can switch within a switching cycle. The longer it stays in a certain state, the greater the loss it causes in this state. For buck converters, the lower D (corresponding lower VOUT), the greater the loss generated by loop 2.
1. Loss of switching device MOSFET conduction loss
The following equation gives a more accurate way to estimate losses, using the integral of the current waveform I2 between IP and IV instead of the simple I2 term PCOND(MOSFET) = ((IP3 - IV3)/3)×RDS(ON)×D = ((IP3 - IV3)/3)×RDS(ON)×VOUT/VIN In the formula, IP and IV correspond to the peak and valley values of the current waveform respectively. As shown in Figure 3, the MOSFET current rises linearly from IV to IP , for example: if IV is 0.25A, IP is 1.75A, RDS(ON) is 0.1Ω, VOUT is VIN/2 (D = 0.5), the calculation result based on the average current (1A) is: PCOND(MOSFET) (use Average current) = 12×0.1×0.5 = 0.050W.
Use waveform integration for more accurate calculations: PCOND(MOSFET) (calculate using current waveform integration) = ((1.753 - 0.253)/3)×0.1×0.5 = 0.089W or approximately 78%, higher than calculated based on average current The results obtained. For current waveforms with a small peak-to-average ratio, the difference between the two calculation results is very small, and the average current calculation can meet the requirements.
2. Diode conduction loss
The conduction loss of MOSFET is proportional to RDS(ON), while the conduction loss of diode depends largely on the forward conduction voltage (VF). Diodes generally have greater losses than MOSFETs, and diode losses are proportional to forward current, VF, and conduction time. Since the diode conducts when the MOSFET is turned off, the conduction loss of the diode (PCOND(DIODE)) is approximately: PCOND(DIODE) = IDIODE(ON)×VF×(1 - D) where IDIODE(ON) is the diode conduction average current during the period. As shown in Figure 2, the average current during the diode conduction period is IOUT. Therefore, for a buck converter, PCOND(DIODE) can be estimated according to the following formula: PCOND(DIODE) = IOUT×VF×(1 - VOUT/VIN) Different from the MOSFET power consumption calculation, using the average current can get a more accurate power consumption calculation result, because the diode loss is proportional to I, not Iout. Obviously, the longer the conduction time of the MOSFET or diode, the greater the conduction losses. For a buck converter, the lower the output voltage, the more power dissipated by the diode because it is in the on state longer.
3. Switching dynamic loss
Since switching losses are caused by the non-ideal state of the switch, it is difficult to estimate the switching losses of MOSFETs and diodes. It takes a certain amount of time for the device to go from fully on to fully off or from fully off to fully on. In this process, power is generated. loss.
The relationship between the drain-source voltage (VDS) and the drain-source current (IDS) of the MOSFET shown in Figure 4 can well explain the switching loss of the MOSFET during the transition process. It can be seen from the upper part of the waveform that tSW(ON) and During tSW(OFF), voltage and current transients occur, and the capacitance of the MOSFET is charged and discharged. As shown in Figure 4, before VDS drops to the final on-state (= ID×RDS(ON)), the full load current (ID) flows through the MOSFET. In contrast, during turn-off, VDS gradually rises before the MOSFET current drops to zero. to the final value in the shutdown state. During the switching process, the overlap of voltage and current is the source of switching losses. This can be clearly seen in Figure 4.
This is easy to understand. As the switching frequency increases (the period shortens), the proportion of switching transition time increases, thereby increasing switching losses. During the switching conversion process, the impact on efficiency of the switching time being one-twentieth of the duty cycle is much smaller than the case where the switching time is one-tenth of the duty cycle. Since switching loss has a great relationship with frequency, when operating at high frequencies, switching loss will become the main loss factor.
Review Editor: Liu Qing
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