Technology Section TDK Power Electronics World Basic Circuits of Insulation Type DC-DC Converters Insulation type DC-DC converters actively use transformers and support high output power. Understanding the basic principles and core circuits will deepen your understanding. RCC Type (self-exciting flyback converter) Low Output Power Types ＜Principles of Transformers and Direction of Electromotive Force＞ Symbol indicating the beginning of the windings of the primary winding Magnetic flux from the primary winding Switch ON When Q1 is ON: When Q1 is OFF: Q1 Base winding Load Inductive electromotive force Reverse electromotive force Core ON/ON Types and ON/OFF Types When the switch is ON, magnetic flux is generated by the primary winding, but electromotive force (reverse electroReverse effect motive force) is generated to prevent the magnetic flux magnetic flux from doubling. The magnetic flux from the primary winding the secondary winding from passes through the core and reverse effect magnetic flux from the secondary winding is generated, creating electromotive force (inductive electromotive force) and current Current (inductive current) flows. When the switch is OFF, the current flows in the opposite direction. Key Point *A gap is placed in the transformer core to prevent magnetic saturation (See page 19). The direction of the electromotive force from the primary and secondary windings (reverse electromotive force and inductive electromotive force) is towards the gray circle ( ). Symbol indicating the beginning of the windings of the secondary winding *RCC : Ringing Choke Converter When Q1 is ON as a result of the base current from the base winding, collector current flows. When the base current is insufficient and Q1 is OFF, current flows on the secondary side. The converter is a self-exciting type that performs this operation repeatedly. It requires only a small number of components and can be used as a simple, low output power power supply. DC-DC converters are available in ON/ON types that output energy when the switching elements are on and ON/OFF types that output energy when the switching elements are off. Types by Output Voltage and Power ON/ON Types (Multi-switching types: Push-pull, half-bridge, full-bridge, etc.) ON/OFF Types (RCC, flyback, etc.) Output voltage (V) Medium to high output power types use multiple switching devices which makes the circuit configuration more complex but enables higher efficiency, lower noise, and advanced functionality. Push-Pull Type Medium to High Output Power Types Low and Medium Output Power Types 1000 100 10 0 0 Switch ON Q1 Transformer + Switch OFF + from the generated magnetic flux (energy storage). The direction of the diode is reversed, so no inductive current flows through the secondary winding. − (Connected to control circuit) lated in the core is released and current flows through the diode (→ ). The transformer coil plays a role similar to that of the choke coil. The transformer core stores energy, so no choke coil is needed. B-H Curves of Magnetic Cores When Q1 is ON: When Q2 is ON: Q1 and Q2 are switched in alternation. Push-pull types are commonly used as power supplies up to about 300 W. Switch ON ❶When the switch is ON, electro- + Reverse electromotive force Switch OFF Choke coil Inductive electromotive force + D2 − Q1 Q3 The transformer is the key component of insulation type DC-DC converters. The narrower the curve, the smaller the losses. Comparison of Performance of Core Types DC output Q2 15 H: Magnetic field The half-bridge type replaces Q1 and Q2 with two capacitors. Magnetic permeability Saturation magnetization Q4 ❷ When the switch is OFF, the choke coil generates (Connected to control circuit) Excitation Process The greater the magnetic permeability, the greater the slope. Magnetic permeability Used as high-efficiency, high output power power supplies with outputs of several hundred watts and higher. motive force (reverse electromotive force and inductive electromotive force) is generated in the primary and secondary windings as a result of the transformer principle and current flows through the diode (D1) (→). At this time, energy is stored in the choke coil. electromotive force, preventing changes in the current, the stored energy is released, and current flows through the reverse flow diode (D2) (→ ). B: Magnetic flux density Saturation magnetic flux density Full-Bridge Type Medium to High Output Power Types Medium Output Power Type D1 1000 Power (W) (single-switching forward, etc.) Q2 ❷ When the switch is OFF, the energy accumu- Key Point 100 ON/OFF Types & ON/ON Types DC output ❶ When the switch is ON, current flows in the primary winding (→) and the core is magnetized 10 Iron losses When Q2 and Q3 are ON: When Q1 and Q4 are ON: Manufacturing cost Silicon Ferrite Amorphous Acceptable Good Excellent Excellent Acceptable Acceptable Poor Excellent Excellent Acceptable Excellent Iron cores generate high losses (thermal losses) at high frequencies, so they are not used. Poor 16