The
optocoupler has a wide range of product types due to its one-way transmission, the input and output are completely electrically isolated and have different functions. It has been widely used in electrical insulation, level conversion, interstage coupling, drive circuit, switching circuit, signal isolation, interstage isolation, pulse amplification circuit, digital instrumentation, pulse amplification, instrumentation and other circuits.
The
frequency converter is mainly composed of rectification, filtering, inverter, control, etc., and in these modules, it also involves driving, detecting, and protecting circuits. Therefore, the use of optocouplers in these circuits greatly increases the reliability of the circuit. It reduces the difficulty and cost of circuit design and is easy to maintain.
Next, a brief description of some of the optocouplers available in the frequency converter.
1. Input Control ModuleThe optocoupler with the output of the ordinary transistor can isolate the operation module from the single chip microcomputer, which can effectively improve the anti-interference and safety performance. The typical models of the transistor output are EL817, EL357N, EL3H7 and other models to choose from.
2. IGBT Inverter ModuleThe IGBT can be directly driven by the IGBT drive optocoupler, making the circuit particularly simple. The optional models are HCPL316J, HCPL3120, PC923, PC929, TLP250 and so on.
3. Detection and ProtectionIn the working process of the inverter, it is necessary to monitor a series of parameters such as current, voltage and temperature in real time. The high-speed optocouplers such as HCPL-7840, TLP559, 6N136 and 6N137 have extremely fast conversion speed, which is very suitable for real-time detection of voltage and current.
The optocoupler 6N137 and the resistor step-down circuit are used to collect the voltage signals of the inverter U, V, W three-phase output to the DC link negative N, so that the three-phase signals become unipolar SPWM voltage pulses, which are convenient for unidirectional optocouplers. match. The unipolar SPWM pulse voltage is filtered by a small capacitor to become a relatively smooth sinusoidal half-wave signal as shown in the figure. It reflects the instantaneous value of the inverter AC voltage (half-wave) and is then sent to the corresponding CPU or ASIC for processing. The instantaneous value of the voltage can be obtained as needed, and the effective value of the voltage can also be calculated. It can meet the needs of control and meet the needs of display measurement. For example, the voltage vector control inverter studied by Sanken Corporation of Japan uses this circuit to measure the AC output voltage, and the control effect is good. At the same time, there are optocoupler applications in power supply phase loss and ground fault detection and overheat detection circuits.
A common method for power phase loss and ground fault detection is to detect the three-phase current balance by a current mutual inductance coil placed on the main circuit (input or output). The schematic diagram is shown in the figure above. The photocoupler cutoff output is normal at 1. When a phase power supply is leaking or missing phase to ground, the photocoupler P512 is turned on and a fault signal is generated because the three-phase current imbalance detecting coil induces a potential.
The heat dissipation system in a slightly larger air-cooled frequency converter is generally composed of a plurality of radiators and is equipped with an axial flow fan. Each heat sink is equipped with a thermal element, such as PTH1~PTH3 as shown in the figure. Some inverters also install a thermal element on the main control board, such as PTH4 shown in Figure 16. The four thermal elements are connected in series and then connected to the optocoupler element EL817. In the normal state, the thermal element is a normally closed contact, and the optocoupler conduction output signal is 0; when the heat sink is overheated, the thermal element is turned off, the optocoupler is turned off, and the output signal is 1, and the signal is turned off by RC filtering. The drive signal of the IGBT notifies the CPU to issue an overheat alarm signal.
In the inverter circuit design process, the transistor output optocoupler (EL817, EL357N), high-speed optocoupler (HCPL-7840, TLP559, 6N136, 6N137), IGBT drive optocoupler (HCPL-316J, HCPL-3120, PC923) , PC929, TLP250), can effectively reduce the design difficulty, reduce costs and improve product reliability.
