Abstract: Introduced one kind of voltage soft switch phase shifting controls the PWM induction heating series resonance invertor, has analyzed the phase shifting pulse width accent merit method, discussed has realized the zero potential switch’s condition, proposed realized the power component ZVS control policy under each kind of loading condition.
Key word: Pulse-duration modulation; Phase shifting; Zero potential switch
Introduction
This article analyzed phase shifting PWM to control series resonance invertor’s realization. Adjusts through the change phase shifting angle transmits for the load power, to the power MOSFET output capacity’s influence, proposed one kind of control plan guaranteed the power component realizes ZVS under each kind of loading condition, guaranteed that in the entire bridge topology the MOSFET flexible cut, causes the turn-on frequency to follow the resonance frequency closely, causes the invertor work in the power factor close 1 accurate resonance condition.
1 phase shifting control series resonance invertor principle of work
Entire bridge construction’s series resonance invertor as shown in Figure 1. 4 switching valve S1~S4, respectively by 50% dutyfactor clear, S1 and S4 are the datum arm switches, S2 and S3 are the phase shifting arm switches, on each bridge arm’s power tube by 180° the phase difference clear and the shutdown, between two bridge arm switch’s driving signal differ a phase angle Phi, control succession as shown in Figure 2. Causes the output between the positive and negative voltage to insert a clamp alternately to zero voltage value, so long as like the change phase angle Phi may change the output voltage correspondingly the effective value, achieves the adjustment output finally the goal. When uses the phase shifting adjusts the merit method, electric circuit’s operating frequency change is small, has the good load compatibility. When specially the loaded impedance has compares the high quality factor, in its accent merit scope the frequency shift is smaller.
Figure 2
When the operating frequency above the resonance frequency, namely the load presents the perceptual condition, load current io falls behind accurate square-wave voltage vAB, as shown in Figure 2. Must realize power tube’s zero potential clear, must have on switching valve’s output capacity’s electric charge which the enough energy uses for to screen out is going to clear; And switching valve’s junction capacitance charge which must shut off for the identical bridge arm. After the switching valve output capacity electric discharge had ended, the diode after flow, will this time give the driving signal, the switching valve in the zero potential condition clear. If the turn-on frequency follows close on the resonant dot frequency maintains constant, because Phi angle enlargement, the load current turns the positive phase before the S3 clear, ZVS will not be able to realize. In order to prevent to lose the ZVS condition, should guarantee the switching valve output capacity’s complete electric discharge, therefore the controller needs to raise the turn-on frequency, goes in the S3 clear to obtain the bigger negative load current. This system has used on electric circuit’s parasitic element, enables in the entire bridge construction 4 switch component to move in the zero potential cut, not only reduced the cut loss and the switch stress, also does not look like hard switch PWM such to need to use the cushion absorbing circuit.
2 phase shifting control series resonance invertor’s analysis
System’s entire bridge construction equivalent circuit as shown in Figure 3. May know following relational by Figure 3 the existence:
Accurate square-wave voltage vAB, approximate sine wave load current io. When works above the resonance point, may obtain ZVS, the mesh current Alpha falls behind by certain phase the circuit of the cells voltage, as shown in Figure 2. The lagging of phase may be represented as
In the formula: Zin is the input impedance.
In circuit of the cells voltage for positive, the mesh current for negative time, the difference Beta is decided ZVS realizes base element. Obtains ZVS βmin the mathematical expression may obtain from the below coulometric analysis: The mesh current must be big enough, can cause to store up in the S3 energy qcoss release, and provides in Beta the scope to the S4 energy, must pay attention to qcoss is also Vin stores up in output capacity Coss the energy. According to these electric quantity’s between relation, βmin may (2) obtain by the type.
In the formula: Ip is the load peak electric current.
Therefore realizes the frequency which ZVS needs is between an output capacity electric quantity and the full load current function; May obtain before losing the biggest phase-shift which the ZVS condition permits, if the type (3) shows.
Investigates ax=2 (α-βmin) (3)
Figure 4 has given in view of the different load value about frequency scale value ωn several φmax curves.
This curve had demonstrated that if uses perfect switch component Coss=0, when namely βmin=0, speaks the biggest phase-shift which to ZVS permits. But says to the practical application, βmin is bigger than zero, φmax is smaller than the ideal condition. Transmits through the computation for the equivalent impedance power may obtain the output expression is
The nominal output frequency expression is
Type (4) and type (5) joint liter
Speaking of the power regulation, the Q value’s size with the biggest phase-shift size which permits corresponds. If expected the output by the power request which assigns defined that for guaranteed output’s constant, works when the low Q value, needs the angle of phase shift possibly investigates ax to be bigger than, like this will lose the ZVS condition. In order to reduce βmin, increases investigates ax, should cause the circulation energy to be as far as possible small, therefore, switch frequency characteristic as far as possible close resonance frequency, in guarantees under the ZVS condition, may through the work in compared to the resonance frequency slightly high frequency rank, namely through cause the load work to get down the solution in the perceptual condition.
3 control objectives and strategy
This system’s control objective is, the adjustment output, and maintains ZVS under each kind of loading condition.
As for the power regulation, if regarding each kind of loading condition, βmin known, one kind may use the type after the turn-on frequency and the phase shifting accent merit method (1), the type (2), the type (3) and the type (6) iterates obtains. Therefore, this article proposed one kind guaranteed realizes the ZVS control policy under each kind of loading condition, contains choice which 3 kind of simplified controls realize, namely fixed Beta phase control; Fixed Beta timing control and along with summit current compensation Beta timing control.
The simple realizes the method is to suit in the worst load condition, namely the adjustment Beta is a constant phase value βfixed. βfixed is guarantees under the ZVS condition, needs βmin the maximum value to the variation of load scope, it may (2) obtain through the type. Was guaranteeing ZVS the lowest Q value place will produce βmin maximum value, because, the mesh current is this time smallest. Assigns Pn, Q and βfixed, may through the type (1), the type (3) and the type (6) iterates obtains ωn and myo- K the choice Pn value must be regarding the variation of load scope in maximum value, thus reduces the circulation energy loss. The big Pn value means that the turn-on frequency will follow closely the resonant dot frequency and big Zo. But this will cause the big clear rate, and reduced the secondary electric capacity peak voltage, thus permission low voltage rank secondary electric capacity’s application.
The analysis indicated that the fixed Beta time’s adjustment only improved invertor’s power factor slightly compared with the fixed Beta phase adjustment. As for the fixed phase control, Alpha is only is slightly bigger than the fixed time control. Along with load parameter’s enlargement, says to the fixed phase control tβ increases, however regarding the fixed time control, tβ and anticipated maintains the constant equally. Therefore, the fixed time control regarding tβ is ideal, namely tβfixed=βfixed/ωn. Transforms when the normalized time as the reality time, tβ needs to be multiplied by. Chooses high Pn is to obtain the higher normalized frequency. Because the invertor will further work nearby the resonance point. Will guarantee ZVS under this condition’s control policy.
Here, we choose along with the summit current compensation Beta timing control way. tβ no longer by a fixed time value, but controls by the change time strategy. In this kind realizes under the way, along with Q value’s enlargement, circuit of the cells summit current Ip will increase along with it. Tβ will possibly reduce regarding the higher summit current, this will have contacting with the load peak electric current Beta to be possible (2) to manifest by the type. A control system ought to be able to realize increases along with the load peak electric current reduces automatically tβfixed the starting value. The control according to type (7) realizes.
tβ=tβfixed-KIp (7)
In the formula: Ip is the summit current;
K is the most ideal gain.
Figure 5 expression, when fixed Q value, uses the summit current compensate control, compares with the fixed time or the fixed phase control method, the power factor had the further enhancement (Alpha relatively small).
4 conclusions
This article had expounded one kind of control policy, proposed obtains ZVS under each kind of loading condition to realize the method. Carries on the output through the change phase shifting angle the adjustment, changes the turn-on frequency to guarantee the ZVS condition. The designer may uses this control method based on the load various requirement and the different application situation.
