This controller incorporates a rotor position decoder for proper rectification timing, a reference level to compensate for sensor temperature, a programmable sawtooth wave oscillator, an error signal amplifier, a pulse modulator comparator, three open-collector top-drive outputs, and three high-current totem-pole bottom-drive outputs ideal for driving power MOSFETs. Furthermore, the MC33035 features underlock capability, cycle-by-cycle current limiting with selectable time-delay latch-up shutdown mode, and internal thermal shutdown. Typical motor control functions include open-loop speed, forward or reverse rotation, and run enable.
DC motors have advantages over AC motors, such as superior speed regulation performance, convenient and smooth speed regulation, and a wide speed range, and are therefore still widely used in many industrial applications. DC speed regulation mainly includes the following methods: armature series resistance speed regulation, armature voltage changing speed regulation, PWM DC adjustment system, dual closed-loop DC speed regulation system, digital DC speed regulation system, and constant power speed regulation by changing the excitation.
DC motor test power supplies must be used for voltage regulation tests, light load tests, load tests, and overload tests according to the testing requirements of DC motors, as well as dynamic process tests. This requires them to have continuous voltage regulation and overcurrent functions, and a fast response speed.
Circuit Design
1. Rectifier circuit calculation
The rectifier circuit uses a three-phase uncontrolled rectifier. This circuit is characterized by its simplicity, speed, and output waveform that meets the requirements of the inverter circuit. The main function of the input rectifier and filter circuit is to convert AC voltage to DC voltage; in addition, it must have a certain output voltage holding capability to prevent interference from the power grid from entering the power supply, as well as interference generated by the power supply itself.
Diodes should be selected based on their effective value, and the calculation process is as follows.
The waveform of the rectified output voltage Ud fluctuates 6 times within one cycle, and the pulsating waveforms are identical. Therefore:
(1)
U2 is the effective value of the phase voltage, which is 220V.
Ud = 2.34 × 220 = 514V (2)
P0 is 50kW, and the efficiency η is taken as 0.8, so P0 = Ud × Id × 0.8, Id = P0 / Ud × 0.8 = 122A.
The effective value of the current flowing through the rectifier diode is:
Design of DC motor drive power supply based on IGBT drive module
(3)
The maximum reverse voltage that each rectifier diode can withstand is:
(4)
Considering the mains voltage fluctuation of -10% to +10%, the minimum parameter of the diode should be:
IA(VT) = I(VT) × 1.1 = 78A
UFM = URM × 1.1 = 593V
Therefore, the diode's rated parameters are 200A and 1200V. Thus, the selected diode is a ZP200-ZL20 bolt-type general-purpose rectifier diode.
2. Calculation of filter capacitor
In a power supply, the power frequency filter is connected between the power frequency rectifier and the inverter circuit. It can not only convert pulsating current into smooth DC, but also suppress high-frequency interference, especially the high-frequency interference generated during conversion.
The filter capacitor should ideally be an electrolytic capacitor with low equivalent series resistance and large capacitance, as the equivalent series resistance directly affects the output ripple voltage. Therefore, to reduce the equivalent series resistance, four capacitors are connected in series and parallel to obtain the required capacitance.
The mains voltage is 380V. Ignoring fluctuations, the no-load DC voltage Ud0 is approximately the peak line voltage of 540V. Under load, the voltage drops slightly; the capacitor voltage is:
Ud = 1.35 × 380 = 513V (5)
The input current Id is 125A, which is the input current of the high-frequency transformer. The DC-side voltage is pulsating under load, as shown in Figure 2. The maximum voltage drop ΔU is considered to be 10%. Within the T1 interval, capacitor C discharges to the load, completing one charge-discharge cycle within the T/6 interval.
Assuming the discharge current is constant during capacitor discharge, then:
Id=C△U/T1 (6)
In the formula, Id is the input current of 125A; ΔU is the voltage change (ΔU = Ud0 × 10% = 54V); T1 is the capacitor discharge time; Ud0 is the no-load DC voltage of 540V; T is the power frequency AC cycle of 20ms; and T1 is the power frequency AC angular frequency. From this, we can obtain:
(10)
The capacitor can withstand a peak line voltage of 540V.
In practice, four 5600μF/400V electrolytic capacitors and two large resistors are connected in parallel as one group, and two groups are connected in series to form a filter capacitor bank. The capacitance is 5600μF, which is greater than the calculated value, and the filtering effect meets the requirement of less than 10%.
Therefore, the DCMCE 1669 type electrolytic capacitor with a specification of 5600μF/400V is selected.
Since electrolytic capacitors are not ideal capacitors, their impedance affects the voltage across the capacitor, and the voltage fluctuates. Therefore, in order to stabilize the voltage across each capacitor and make the voltage across each group of capacitors equal, a voltage equalizing resistor R2 and R3 are connected in parallel across each group of capacitors, with R2=R3=30kΩ.
As for the capacitor C1 used to filter out high-frequency interference, its capacitance is difficult to determine because high-frequency interference includes interference from the power grid and interference from the power supply. Therefore, you can try to select C1=(2.5±5%)μF or other capacitors of the same order of magnitude, as long as the peak voltage of capacitor C1 meets the requirements. The peak voltage Up=600V.
When an AC voltage is applied to a capacitor-input rectifier and filter circuit, the capacitor charging often causes a large inrush current. Therefore, a current-limiting resistor R1 of 20Ω/20W is selected. After startup, after a delay, switch S1 closes, removing R1 from the main circuit.
3. Inverter circuit calculation
A full-bridge inverter circuit was selected for the inverter circuit, using IGBTs as the switching devices.
IGBTs combine the advantages of MOSFETs and GTRs, featuring the high input impedance, good thermal stability, and low drive power of power MOSFETs, while also possessing the advantages of low on-state voltage, low conductivity loss, and high withstand voltage of GTRs.
Calculations are performed on the main circuit, and IGBTs are selected according to the design objectives.
The function of an IGBT is to convert DC voltage into a square wave voltage through its periodic switching action; it is a key component of the inverter circuit. Because it is relatively fragile, its design and selection directly affect the safety and reliability of the entire system. Therefore, the selected parameters must be within its forward bias safe area (FBSOA), and a large margin should be allowed when calculating the parameters.
① Rated voltage
The maximum DC output voltage after rectification and filtering of the input grid voltage:
(11)
Where Ud is the maximum steady-state voltage that the IGBT can withstand, U is the effective value of the grid voltage (380V), 1.1 is the fluctuation coefficient, and α is the safety factor (taken as 1.1).
The peak voltage Uce at shutdown is 988V. The rated voltage should be taken upwards, and the actual voltage level value should be taken as 1200V.
② Rated current Ic
High-frequency transformer primary side current
I1 = I2 × N2 / N1 = 125A
Where I1 is the primary current of the high-frequency transformer, I2 is the output current of the test power supply, and N1 and N2 are the number of turns on the primary and secondary sides of the high-frequency transformer.
The average current I on each IGBT is 63A.
The rated current Ic is the rated value given in the IGBT manual at a junction temperature of 25°C, Ics=186A.
Where Ics is the calculated rated current of the IGBT; I is the average current on each IGBT; 1.414 is the peak current factor; 1.5 is the Imin overload capacity factor; and 1.4 is the Ic reduction factor. The rated current Ic is taken as 200A based on the tube's current rating. In summary, the rated voltage of the IGBT is 1200V and the rated current is 200A. Therefore, the Siemens high-power IGBT module BSM200GB120DLC is selected.
4. Output Rectifier Circuit Design
Switching rectifier diodes should not only have short reverse recovery times and small reverse recovery currents, but also preferably slow reverse current recovery speeds to reduce noise. Commonly used types include gold-doped diffused diodes, epitaxial diodes, Schottky diodes, and fast recovery diodes (PIN diodes). Among them, fast recovery diodes are characterized by a low forward voltage drop (0.85V at room temperature, decreasing further with increasing junction temperature, reaching only 0.6V at 150℃, similar to Schottky diodes); a short reverse time (no more than 200ns); and a reverse leakage current of only 1mA at 150℃ and rated voltage, close to that of ordinary rectifier diodes. Therefore, fast recovery diodes are preferred.
For a single-phase full-wave rectifier circuit, the rated current of the rectifier diode is:
IN = 0.5 × I² = 500A
In the formula, IN is the rated current of the rectifier diode; I2 is the power supply output current of 1000A.
Maximum back pressure on the pipe:
Um = 2 × U2 = 136V
U2 represents the output voltage amplitude of the test power supply, which is 68V. Considering a certain margin, the diode was selected based on a voltage of 300V and a current of 2000A. The final choice was the ZK300-35ZT3 planar fast recovery diode.
Control circuit design
In this design, phase-shift control is used in the control circuit. The phase-shift PWM controller enables the four switches of the full bridge to conduct in turn by phase shifting. During the process of two switches in the same bridge arm conducting in turn, the leakage inductance of the transformer and the output parasitic capacitance of the switch form a resonant cavity, which allows the voltage on the capacitor to discharge at the fastest speed, ensuring that the switch is in a zero-voltage switching state (ZVS), thereby avoiding the overlap of voltage and current during the switching operation.
PWM phase-shift control is implemented using the error amplifier of the UC3875. A PI regulator is connected to the four pins of the UC3875, and a voltage sensor compares the output voltage with a given reference voltage to control the phase between A, B, C, and D, ultimately adjusting the waveform duty cycle to stabilize the power supply at a predetermined value. The A/B and D/C half-bridges can be individually controlled for conduction delay (dead time). During this dead time, the output capacitor of the next power switching device is ensured to discharge completely, providing the voltage turn-on condition for the upcoming switching device.
Drive circuit design
With the widespread application of IGBTs in various power conversion devices, the selection and performance of IGBT driver modules are attracting increasing attention from users. An ideal driver module should not only provide sufficient turn-on and turn-off gate voltages for the IGBT, but also possess rapid and reliable protection functions, while striving for circuit simplicity and stability. There are many commonly used IGBT driver modules, among which the EXB series is the most widely used. The EXB841 driver module is widely used in switching power supplies, UPS, power drives, and power compensation.
IGBTs require a +15V voltage to achieve a low turn-on voltage during switching, and also a 5V gate-off voltage to prevent malfunctions during turn-off.
Both voltages can be generated by the internal circuitry of the 20V-powered driver.
