Abstract: Introduced and has compared the voltage control and the current control DC/DC converter’s basic principle, designed based on current control PWM the control chip UC3846 high efficiency DC/DC converter’s practical electric circuit, proposed two UC3846 output pulse lock mode, designed one kind of novel IGBT driving circuit, the experimental result proved that this electric circuit had the good control characteristic and the stability.
Key word: DC/DC converter; Pulse-duration modulation; Voltage control; Current control; IGBT actuation introduction
Introduction
Along with application domain technology and so on industry, aviation, astronautics, military unceasing development, the people are also getting higher and higher to the switch voltage-stabilized source’s request. Some system the request which proposed to the high efficiency switch voltage-stabilized source is: The input voltage is AC220V, the output voltage is DC38V, the output current is 100A. Switching power supply’s structure to carry on AC/DC then again DC/DC first generally the form, considered the paper length the limit, only carries on the discussion to the DC/DC mapping division.
The high efficiency DC/DC converter main circuit analysis situs has many kinds, such as double barrel is stirring up-like, the push-pull type, the semi-bridge system and entire bridge-type and so on. The control chip’s type are also many, mainly divides into the current control and the voltage control two broad headings. The voltage control only to the output voltage sampling, carries on the closed-loop control as the feedback signal, uses the PWM technology adjustment output voltage, looked from control theory’s angle, this is one kind of simple ring control system. The current control is in the voltage control foundation, increases an electric current negative feedback link, causes it to become the Shuanghuan control system, thus enhanced power source’s performance.
Figure 1
According to each kind of analysis situs and control mode’s technical mature degree, the project realizes the difficulty, target and so on electrical specification as well as cost comparisons, this article selects the semi-bridge system DC/DC converter to take the main circuit, current mode PWM control chip UC3846 takes this system’s control unit.
1 voltage control PDM keyer and current control PDM keyer [1]
Figure 1 is the voltage control converter’s functional block diagram. Power source output voltage’s sampling feedback value Vf and reference voltage Vr carries on quite enlarges, obtains error signal Ve, it after saw-tooth wave signal comparison, the PWM comparator outputs the PWM control signal, actuates the switching valve make-and-break after the driving circuit, produces the high frequency square-wave voltage, by the high-frequency transformer transmission to the vice-side, obtains the voltage which after the rectification filter needs. The change voltage assigns Vr, then changes output voltage Vo.
Figure 2 is the current control converter’s functional block diagram. The permanent frequency clock pulse setting R-S latch, outputs the high level, the switching valve breakover, the transformer primary side electric current linearity increases, when electric current achieves Ve when sampling resistance Rs pressure drop Vs, the PWM comparator turns over, outputs the high level, the latch repositions, the driving signal changes low, the switching valve shuts off, causes the R-S latch setting until the next clock pulse. The electric circuit is this one by one examination and the regulating current pulse.
Figure 2
When the mains input voltage and/or the load changes, two kind of control type’s dynamic response speed is different. If voltage magnification, then switching valve’s electric current rate of rise changes quick. Speaking of the current control, as soon as so long as the current pulse achieves the hypothesis the peak-to-peak value, the pulse width comparator on the movement, the switching valve shut off, has guaranteed the output voltage stability. Speaking of the voltage control, examines the electric circuit to the electric current change not direct reflection, after the output voltage when has changed, only then adjusts the pulse width, as a result of filter circuit’s lagging effect, after this kind of change needs many cycles, can display, obviously the dynamic response speed must be much slower, and output voltage’s stability also comes under certain influence.
Moreover, the current control converter may also realize one by one pulse control and the multi-power source’s parallel operation very easily, and has suppresses the high-frequency transformer bias ability. But when application current control must pay attention to the following question:
- when - the dutyfactor is bigger than 50%, the control circuit is unstable, by must add the pitch compensation;
- the pneumatic control regulation circuit is based on the signal which obtains from the inductance electric current, therefore, power part vibration easily noise introduction control circuit.
2 current control PDM keyer UC3846 introduces [2]
This system used UC3846 to take the switch controller. UC3846 is one kind of double-end output current control PDM keyer chip, its internal structure block diagram as shown in Figure 3 [2]. It draws out foot 1 as current limiting level establishment end; The foot 2 are the voltage reference out-ports; The foot 3 examine amplifier’s opposition input end for the electric current; The foot 4 examine amplifier’s synchronism input end for the electric current; The foot 5 are the erroneous amplifier’s synchronism input ends; The foot 6 are the erroneous amplifier’s opposition input ends; Foot 7 for erroneous amplifier feedback compensation; The foot 8 are oscillator’s external connection electric capacity ends; The foot 9 are oscillator’s external connection resistance ends; The foot 10 are the synchronized ends; The foot 11 are PWM the pulse A out-port; The foot 12 are the places; The foot 13 are the collector supply ends; The foot 14 are PWM the pulse B out-port; Foot 15 for control supply input end; The foot 16 to close the dead end.
By Figure 3 may see that UC3846 is 3 current measurement amplifier (its input voltage must <1.2V) obtain the inductance electric current or the switch electric current signal through an enlargement factor, its output meets the PWM comparator’s synchronism end. When sample amplifier input signal >1.2V, the current mode controller shuts off the time delay. Voltage error amplifier’s output after diode and 0.5V bias songzhi PWM comparator’s opposition end, its output both as demand signal, simultaneously and by current limiting level establishment foot (foot 1) clamp in V1 0.7V, thus has completed one by one pulse current limiting goal. What manservant dynamic electricity class examination amplifier examination is the switch electric current, but is not time the inductance electric current, as a result of the switching valve parasitic capacity electric discharge, the examination electric current will have a big peak front, possibly will cause the electric current to examine the measuring chain to save with the PWM electric circuit misoperation, therefore, should add the RC filter in the electric current examination input end.
Figure 4
UC3846 has the fast protection function, it and the electric current sampling circuit time delay shutdown is different. The protection function foot (foot 16) after examines the amplifier to meet the thyristor the gate extremely, when the electric circuit occurs exceptionally presents the overflow, causes the foot 16 electric potentials to rise 0.35V, the protection circuit movement, the thyristor breakover, causes the foot 1 level to pull to the avenue of approach level, the electric circuit enters the protection condition, the output pulse blockade.
3 high efficiency DC/DC converter’s realization
3.1 DC/DC converter main circuit
Because this DC/DC converter’s input voltage is high, main circuit selection semi-bridge system analysis situs [2][3], as shown in Figure 4. V1, V2, C3, C4 and main transformer T composition semi-bridge system DC/DC transfer network. CT is the current transformer which the primary flow examination uses. C5 is prevents the transformer bias to separate the straight electric capacity. The transformer vice-side uses the full-wave rectification to add on two levels of filter to satisfy the low output ripple the request. R1, C1, R2, C2, R5, C6 and R6, C7 are the absorbing circuits. R3 and R4 play guarantee electric capacity C3 and the C4 differential pressure even role. Resistance R7 and R8 are the output voltage sampling resistances.
3.2 control circuits
Figure 5
As semi-bridge system switching power supply’s control circuit as shown in Figure which the main primary device composes 5 take UC3846. In chart, R1 and C1 constitution oscillator, oscilation frequency f=. In order to prevent in the main circuit at the same time V1 and V2 the breakover, must establish the dead time which the switching valve shuts off. Dead time by oscillator’s drop along decision, this electric circuit’s dead time td=145C1[12/(12-3.6/R1)][4]. R2 and the C2 composition pitch compensating network, guarantees control circuit’s stable [5]. C5 realizes the soft start. May see when foot 1 electric potential <0.5V by Figure 3 non-pulse width output. As shown in Figure 5, the foot 1 passes through electric capacity C5 to arrive, after starting, along with electric capacity’s charge, when the electric capacity voltage is higher than 0.5V only then has the pulse width output, and elevates the pulse along with the electric capacity voltage to broaden gradually, completes the soft start function. Feedback voltage which comes to the main circuit, has been composed the electricity pressure ring’s PI regulator by C3 and R5 and the voltage error amplifier. Moreover, the system also has the perfect protection circuit.
When the system input voltage has pressed or the undervoltage (/undervoltage judgment electric circuit omitted), has been possible to cause Figure 5/the undervoltage input end is the low level, light pair OP1 outputs the high level, therefore, 16 exerts the pulse through speed-up capacitor C6 and diode D6 to the UC3846 foot, will thus cause the UC3846 chip interior thyristor breakover which shown in Figure 3, will cause the foot 1 level through the internal circuit to pull to the avenue of approach level, the electric circuit enters the protection condition, the UC3846 chip output pulse blockade. Moreover, the light pair OP1 output’s high level causes the triode Q407 saturated breakover earth. Because electric capacity C6 the acceleration, triode Q407 compared to after fore-mentioned thyristor breakover slightly late. Because triode’s breakover pressure drop is smaller than the thyristor breakover pressure drop, the thyristor cannot maintain the breakover is the thyristor resumes the shutdown. After working as/the undervoltage breakdown to eliminate, triode Q407 closure, system again output pulse.
Figure 6
When overflow or overload, comparator LM393 outputs the low level, light pair OP2 outputs the high level, adds through D7 in the foot 16, similarly will block the pulse output. As a result of thyristor maintenance breakover, therefore system, when the overflow does not overload, must start can have the pulse output.
3.3 driving circuits
IGBT is one kind of voltage control component, (for example GTR) compares with the current control component, IGBT has the driver power to be small, shutter speed quick characteristic, therefore, IGBT obtained the rapid development in recent years in the variable current technology’s application. IGBT has the special-purpose actuation chip, like Fuji Corporation’s EXB851 and EXB841, Mitsubishi Corporation’s M57959L and so on, these driving circuit has the turn-on frequency to be high, the driver power is big, merits and so on overflow protection, but must add the extra actuation power source, and the price is high, makes the facility cost to enhance greatly. But the pulse transformer has the volume to be small, the price cheap, does not need the extra actuation power source, thus obtains the widespread application.
But when direct drive, because its pulse front and the back edge are not very steep, causes IGBT to clear and the shutdown speed comes under certain influence.
Shown in Figure 6 the IGBT driving circuit has the turn-on frequency to be high, the driver power is big, the structure is simple, the negative pressure shuts off, merits and so on price small advantage.
When the IGBT capacity is small, the UC3846 foot 11 and the foot 14 may the direct drive pulse transformer. When the IGBT capacity is big, the UC3846 driving force is insufficient, V11~V14, D11~D14 constituted the pulse transformer’s driving circuit. D9 and the D10 function is mainly helps V11~V14 the shutdown, if does not have time D9 and D10, when PWM1 is the high level, when PWM2 is the low level, V11 and the V14 breakover, PWM1 and PWM2 are afterward the low level, the energy which in the pulse transformer leakage reactance stores up after D12 and the V14 after flow, A electric potential drops to - 0.7V, even if PWM1 is the low level, V11 the breakover, finally burns down V11, in addition the D10 goal is lets in the electric circuit D12 and V14 when the after flow A electric potential immobilization in 0V, thus is advantageous to V11 or the V13 shutdown; Likewise, the D9 function is advantageous to V12 or the V14 shutdown.
4 experiments and conclusion
Develops a project prototype according to the above design mentality. When the input DC voltage is 250V, the load current is 50A, obtains the IGBT slaving voltage profile and high-frequency transformer primary side voltage waveform as shown in Figure 7. This converter has input, the undervoltage, functions and so on output overflow protection, the output voltage power source relative regulation ≤1%, the load relative regulation ≤1%, output voltage ripple <50mV, has satisfied the design requirements.
