Induction heating power control circuit
The composition of the power electronic control circuit in the induction
heating power supply shows diversified composition methods. The control scheme
is mainly based on the power adjustment method of the induction power supply and
the heating load characteristic requirements. The structure of the control
circuit will be different.
The power control and adjustment methods of induction heating power
supplies can generally be divided into two types: DC side power adjustment and
inverter side power adjustment. DC side power regulation is divided into
three-phase fully controlled rectifier power regulation and DC chopper voltage
regulation and power regulation. The control circuit scheme for power regulation
on the inverter side is based on the requirements of the heating process
characteristics. The control methods that can be used are more flexible.
Commonly used ones include frequency modulation (pFM), phase-shift power
modulation (pSM), and pulse width modulation constant frequency power modulation
(pWM). , pulse density modulation power modulation (pDM), width modulation plus
frequency modulation power modulation (pWM+pFM), pulse width modulation plus
pulse density modulation power modulation (pWM+pDM) and other power modulation
methods.
Induction heating power supplies have the highest heating efficiency and
fastest speed for metal materials, and are low consumption and environmentally
friendly. It has been widely used in thermal processing, heat treatment, thermal
assembly, welding, smelting and other processes of metal materials in various
industries. Induction heating power supply consists of two parts. One part is
the AC power supply that provides energy, also called variable frequency power
supply; the other part is the induction coil that completes the electromagnetic
induction energy conversion, called the inductor.
Basic composition and principles of induction heating power supply control
circuit
(1) The control method depends on the load characteristics of the induction
heating power supply and the power adjustment method. Generally, voltage
feedback control and current feedback control can be used.
1) Using voltage control, the purpose is to ensure that the output DC bus
voltage is constant, that is to say, the terminal voltage applied to the
induction heating winding is constant. The control sample can be taken from the
DC bus voltage or the voltage on the inverter inductor winding or resonant
compensation capacitor. An isolated voltage sensor (TV) is generally used for
sampling. After calculation and comparison processing, the conduction angle of
the thyristor or the phase shift or pulse width of the pWM drive pulse of the
inverter switch tube is controlled to change the DC output to the inverter. The
voltage on the DC bus may change the average (or effective value) of the
inverter output voltage, and ultimately the output voltage is maintained
constant due to the effect of closed-loop negative feedback. The fluctuation of
the input voltage has a great impact on the output power of the heating power
supply, that is, on the heating temperature of the workpiece, and will directly
affect the product process quality requirements of the heated workpiece.
The output power of the heating power supply is p=u2/Z. Under the condition
of unchanged load, the power p is related to the voltage group or the voltage on
the resonance compensation capacitor. Proportional to the square of u. That is,
the heating temperature is proportional to the square of the voltage. If the
voltage is unstable, the heating temperature will be uneven. For occasions where
high temperature stability is required for heating and quenching of blank
workpieces, automatic voltage stabilization function must be provided, otherwise
product quality will not be guaranteed.
2) Using current control, the purpose is to ensure constant output DC or
high-frequency output current. The control sample can be taken from the DC bus
current or the current in the inverter induction heating winding. Sampling
generally uses an isolated current sensor (TA). The object controlled by the
current feedback signal is the same as the voltage control. The purpose is to
achieve changes in the output current, that is, changes in the output power p
and heating temperature. This is because p=IUuzuzu, so it can be seen that
changes in voltage U or load impedance Z will cause changes in current I, that
is, changes in power or heating temperature.
3) Power control is used to ensure constant power output of the induction
heating power supply. The sampling signal simultaneously samples voltage and
current signals. After being processed by the multiplier, the output of the pI
regulator is compared with the power given to control the conduction angle of
the thyristor or the width and phase shift of the inverter drive pulse signal,
or use dynamic impedance. The matching method controls the equivalent impedance
on the power supply side to be equal to the load to achieve constant power,
ensure that the heating temperature is constant at a given power, and meet the
workpiece heating process characteristics and quality requirements.
(2) The control circuit of an induction heating power supply that adopts
the DC side power adjustment scheme requires a phase-locked frequency automatic
tracking system. none. Whether the inverter uses pulse width modulation (pwM)
control technology to adjust power, or uses phase shifting (pSM) to adjust
power, if the inverter side does not perform automatic frequency adjustment, two
major problems will occur: ① Inverter The switching power device cannot work
well in the soft switching state. The switching device is subject to large
voltage and current stress. In addition to endangering the safety of the device,
the switching loss also increases; ② Because the operating frequency of the
inverter is different from the inherent resonant frequency of the resonant
circuit. Not equal, a large reactive current flows through the inverter circuit
or switching device, and the power factor drops, the maximum power output cannot
be reached, and the efficiency of the inverter drops. The purpose of frequency
tracking is to ensure that the switching frequency fs of the inverter is equal
to the natural resonant frequency of of the resonant circuit, and that the
voltage and current are consistent in phase. Because there is no phase
difference between voltage and current, the power factor cos=1 and maximum
power output is obtained.
The phase-locked frequency automatic tracking control circuit can be
implemented using analog circuits, for example: ① Through inverter output
current detection sampling, zero-crossing detection, shaping, etc., it is
processed into a square wave signal to control a dedicated pulse width modulator
(pWM) integrated circuit The synchronization terminal; ② After the current
signal is sampled, it is rectified and calculated by the pI regulator. To
control the frequency setting end of the pWM integrated circuit. The commonly
used SG3525pWM integrated circuit with more complete functions uses an analog
control circuit to realize automatic frequency tracking, which is more
convenient to use. Terminal 6 of SG3525 is the frequency resistor setting pT
terminal. Changing the voltage (or current) of this terminal can change the
frequency of the pWM output pulse. By adding the square wave pulse processed
after current sampling to the synchronization terminal 3 of the pWM modulator
SG3525, the frequency of the pWM output pulse can be synchronized with the
frequency of the current.
Phase-locked frequency automatic tracking control circuits are currently
more commonly used dedicated phase-locked loop integrated circuits, such as
general-purpose CD4046, high-speed phase-locked loop MM74HC4046, and digital
phase-locked frequency tracker control using microcontrollers and digital signal
processors DSp. Systems, etc., digital phase-locked loops and analog
phase-locked processors, DSp digital phase-locked frequency tracker control
systems, etc., digital phase-locked loops and analog phase-locked.
(3) Load matching control is also a key control technology for induction
heating power supplies. According to electrical engineering principles, when the
output impedance of the power supply is equal to the load impedance, the maximum
power can be obtained from the load. The load impedance will change during the
induction heating process. If the output impedance of the power supply cannot be
adjusted in time to match the load impedance, it will be impossible to obtain
the maximum rated output power on the load, and the efficiency of the heating
power supply will decrease. In addition, if the same induction heating power
supply is used to heat workpieces with different load characteristics, and for
the convenience of operation and cost savings, the parameters or characteristic
impedance of the heating power supply load resonant circuit are not changed,
which is common in practical applications. If you adjust the output impedance of
the heating power supply in a timely manner for different heating loads to match
it, you can also obtain maximum power and efficiency on the load for different
heating loads.
The commonly used method for load impedance matching is to use a matching
transformer between the heating power supply and the load, and adjust the change
of the transformer to achieve the purpose of equalizing the equivalent impedance
on the power supply side and the load impedance. Secondly, electronic circuit
control methods can be used to achieve load impedance matching. For example: by
adjusting the pulse density of the inverter switch tube drive signal, the
equivalent impedance on the power supply side is adjusted to match the load
impedance; by adjusting the phase shift of the inverter switch tube drive signal
pulse, the equivalent impedance on the power supply side is adjusted. Match it
to the load impedance. Matching impedance actually means matching power so that
the power supply reaches maximum efficiency.
(4) The control system of induction heating power supply should also
include inevitable failures. Therefore, the heat-sensing power supply, like
other electronic equipment, may also experience occasional malfunctions: In
order to ensure that the power supply meets the requirements for use under the
condition that the electrical performance parameters and technical indicators
meet the requirements, in order to ensure that the power supply operates in
harsh environments and sudden failures. To ensure the safety and reliability of
the power supply device, multiple fault protection functional circuits must be
designed. Once a fault occurs, the power supply device is safe and reliable.
Multiple fault protection circuits must be designed to function so that the
power supply automatically enters the protective working state, or automatically
shuts down and stops working. , or automatically correct the parameters to
operate under a normal and reasonable working condition. It should be said that
among the many quality indicators of induction heating power supply, safety and
reliability are the primary principles.
What should the induction heating power supply protection and control
function circuit include?
1) Overcurrent detection and protection. This includes over-current
detection and protection of input current and inverter output current.
When overcurrent occurs, the protection control circuit should promptly
correct parameters to make the power supply operate in a current-limiting state
and ensure that the power supply operates within the rated input or output
current. Only in the event of severe overcurrent or short circuit failure,
shutdown protection measures can be taken.
2) Input voltage overvoltage and undervoltage protection. If the input
voltage is too high or too low, the main hazards caused to the heating power
supply are:
The voltage or current stress endured by the power device will exceed the
normal rating, causing the possibility of device damage. Moreover, when the
input voltage is too low, the operating DC voltage of the control circuit
exceeds the minimum voltage or even if it does not exceed the minimum voltage,
it will cause the control circuit to operate unstable and cause the heating
power supply to malfunction.
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