Texas Instruments Incorporated Power Management 同步整流 …

1 Texas Instruments Incorporated Power Management Synchronous rectification boosts efficiency . by reducing Power loss Anthony Fagnani (TI) . By Anthony Fagnani Example inputs for this system are a USB port or a lithium- Power Applications Engineer ion (Li-Ion) battery pack with two or three series cells. The DC/DC Power supply steps up the voltage for charging 2 . Introduction a two-cell Li-Ion battery or the battery of a tablet PC. The . Some applications require the highest possible Power effi- other application boosts the voltage of a system Power rail ciency. For example, in a harsh environment that requires . to a high output voltage that can operate at higher duty DC/DC . a DC/DC Power supply to operate in high ambient temper- . cycles where the output voltage 12 Vis.

2 Much higher than the atures, low- Power dissipation is needed to keep the junc- . input voltage. An example input is a 12-V Power rail. The .. tion temperature of semiconductor devices within their high output voltage may be needed for Power amplifiers, . rated range. Other applications may have to meet the industrial PCs, or pump-and-dump energy storage for strict efficiency requirements of ENERGY STAR specifi- higher energy density.. cations or green-mode criteria. Users of battery-operated To evaluate the benefits of synchronous rectification, TI TPS43060/61 . applications desire the longest run time possible, and each application is tested with a real circuit to compare . reducing the Power loss can directly improve run time.. efficiency and Power loss.

3 The TPS43060/61 synchronous Today it is well known that using a synchronous rectifier boost controllers from Texas Instruments (TI) are used to . can reduce Power loss and improve thermal capability. demonstrate MOSFET TIthe synchronous TPS40210 designs. These current- . Designers of buck converters and controllers for step- mode boost controllers integrate the control and gate- . down applications are already employing this technique. drive circuitry for both low-side and high-side MOSFETs. Synchronous boost controllers also have been developed TI's TPS40210 current-mode, low-side-switch boost con- . to address Power efficiency in step-up applications. troller is used for the nonsynchronous designs. 1 . Typical application Basic operation MOSFET Q1 L1.

4 Two typical boost applications can be used to demonstrate A typical block diagram for a step-up (boost) topology is . the difference between synchronous and nonsynchronous shown inC1.. Figure MOSFET. 1. This topology consists of the low-side rectification. The first is a lower-input-voltage application Q2 . Power MOSFET (Q1), the Power inductor (L1), and the USB 2 3. that may operate at low duty cycles or, in other words, output capacitor (C1). For a synchronous topology, the Li-Ion when the output voltage is close . to the DC/DC . input voltage. high-side MOSFET (Q2) is used for the rectifying switch. 1 . Figure 1. Synchronous and nonsynchronous boost circuits Synchronous Rectifier Control DBOOT CBOOT. VCC. Q2. L1. VIN VOUT. IIN D1 IOUT. Q1 C1. Control 9.

5 Analog Applications Journal 2Q 2013 High-Performance Analog Products Power Management Texas Instruments Incorporated In a nonsynchronous boost topology, a . Power diode (D1) is used. Figure 2 shows 2 . Figure 2. Ideal voltage and current waveforms in a boost circuit D1 2 . the equivalent waveforms for the voltage . and Q1 . current through the switches and inductor. During the ON time of Q1,VOUT. the Control OFF ON OFF ON. inductor current ramps VIN up, and VOUT is dis- connected from VIN. The output capacitor . must supply the load during this time. VOUT.. During the OFF time, the inductor current VQ1. ramps down and charges the output capaci- . tor through the rectifying switch. The peak . current in the rectifier is equal to the peak VOUT. current in the switch.

6 VQ2 or VD1. Selecting the rectifying switch . Nonsynchronous controllers use an external . Power diode as the rectifying switch. Three I L1 IIN.. main considerations when selecting the VIN /L1 (VIN VOUT )/L1. Power diode are reverse voltage, forward Slopes current, and forward voltage drop. The . reverse voltage should be greater than the . output voltage, including some margin for IIN. I Q1. ringing on the switching node. The forward . current rating should be at least the same . as the peak current in the inductor. The . forward voltage should be small to increase IIN. efficiency and reduce Power loss. The aver- IQ2 or ID1.. age diode current is equal to the average MOSFET. output . current. N . The package ofMOSFET. the diode . chosen must be capable of handling the.

7 Power dissipation.. Synchronous controllers control another MOSFET for the With a synchronous rectifier, there are two main sources . rectifying . If an n-channel MOSFET is used, a volt- . of Power dissipation conduction and dead-time loss. MOSFET. age higher than the C output voltage must be generated for When the low-side switch turns off, there is a time delay BOOT DBOOT RDS ON 2 D . the gate driver. A bootstrap circuit is used to generate this (t DELAY) before the high-side switch turns on. During this Q1 3 PQ2 . voltage. Figure 1 also includes the typical block diagram delay, the body diode (VSD) of the high-side switch con- VaCCstandard for . bootstrap circuit consisting of the bootstrap ducts current. Typically this is referred to as dead time.

8 Capacitor (C BOOT) and the bootstrap diode (DBOOT. Q1 VOUT).+V. During CC . When the high-side switch is turned on, there is also con- the ON time of Q1, the bootstrap capacitor is charged to a duction loss due to the RDS(ON) of the MOSFET. Equation 2.. regulated voltage (VCC), which typically is regulated by a calculates the duty cycle (D), and Equation 3 estimates . low-dropout regulator internal to the controller. When Q1 the losses (PQ2): turns off, the voltage across the capacitor to ground is VOUT VIN. OUT + VCC, and the required voltage is available to turn on V D= (2). VOUT. the high-side switch. The control circuitry also must be . more complicated to ensure that there is enough delay I2. I . before the rectifying switch turns on to avoid both switches PQ2 = OUT R DS(ON) + VSD OUT 2 t DELAY fSW (3).

9 Turning on at the same time. If this occurs, the output volt- 1 D V IN . 1 D . age shorts to ground through both 1switches, causing high V IN . In an application requiring a low duty cycle, the rectify- currents that can damage the switches. ing switch conducts1for . a larger percentage of each Power loss of the rectifying switch switching period. However, the Power loss in a nonsyn- . To compare the efficiencies of the two different rectifiers, chronous rectifier in a boost topologyFET. is independent of the Power dissipation should be calculated. In the nonsyn- duty-cycle changes caused by variations in VIN. This is . chronous topology, the Power dissipation in the rectifying because variations in VIN also cause an equal but opposite . change in the current the diode conducts.

10 The rectifier Power diode is estimated with Equation 1: tDELAY loss is simply the forward voltage drop times output cur- . PD1 = IOUT VFWD (1) rent per Equation 1. With a synchronous rectifier, there is VSD . some dependence on the duty cycle for Power dissipation 10. High-Performance Analog Products 2Q 2013 Analog Applications Journal Texas Instruments Incorporated Power Management because the conduction losses are caused by . 3 . Figure 3. Measured efficiency and Power loss in a the resistance of the FET. This is unlike a where low-duty-cycle application diode, the are losses by caused the . forward voltage drop. A resistive conduction . loss varies with current TPS43061. squared, leading to a 100 5. Synchronous dependence on duty cycle, with a higher duty 98 TI CSD86330Q3D Efficiency cycle increasing the conduction Power loss.