How is the load box circuit designed
Issuing time:2021-06-11 14:46
The circuit of the electronic load box mainly includes four parts: power MOS tube, fixed resistance, PWM driving circuit and sampling circuit. Because the power of a single MOS tube is limited, it is difficult to meet the power design requirements. Therefore, the MOS tube series fixed resistance design is adopted. The fixed resistance is controlled through the MOS tube driving circuit to adjust the output current, that is, the load box is composed of MOSFET series fixed resistance.
Due to the different working voltages of the load box, two kinds of MOS tubes are required. For dc28v, n-channel enhanced FET | rf540n is selected, with a maximum withstand voltage of 100V, a maximum working current of 33A and a maximum power of 130W at room temperature (25 ° C); Dc270v adopts n-channel enhanced FET ipw60r037p7, with a maximum withstand voltage of 650V, a maximum working current of 76a and a maximum power of 255w at room temperature (25 ° C). The fixed resistance adopts alloy resistance, which is characterized by high heat dissipation performance, strong stability and long service life. Nickel chromium wire with high melting point is selected as the resistance wire inside the resistance, and the resistance shell is made of stainless steel with strong corrosion resistance. High temperature magnesium oxide is selected as the filler between the resistance wire and the resistance shell. The load box is characterized by good heat conduction on the premise of meeting the insulation. The resistor is fixed on the stainless steel heat sink to achieve efficient heat dissipation through high-speed fan.
Due to the difference in the production process of the load box, even the resistance and inductance inside the MOS tube of the same model will have individual differences. Therefore, each branch has a sampling circuit and MOS tube PWM drive control circuit. The schematic diagram of single branch PWM drive can monitor the current of each branch by detecting the jade at both ends of the sampling resistance R8 in Figure 15.
Because the accuracy and stability of the current sampling resistance also directly affect the measurement accuracy, the metal film resistance with high stability is selected as the current sampling resistance, and the load box adjusts the response MOS tube output impedance through the PWM signal through the integral circuit, so as to adjust the output current, which can effectively prevent the MOS tube from bursting due to overcurrent.