Method for Detecting the Cutting Off Signal of BJT in an Electronic Ballast and the Electronic Ballast

Technical Field

This invention relates to a method for detecting the real cutting off signal of BJT (bipolar junction transistor) in an electronic ballast and to a corresponding electronic ballast employing said method.

Background Art

Figure 1 is part of circuit of a prior art electronic ballast that is most pertinent to the present invention. Said electronic ballast comprises a driving transformer Tl, two base units, and an upside BJT Sl and a downside BJT S2 that form a half-bridge circuit, wherein a center point M of the half- bridge (HB) circuit is defined between BJT Sl and BJT S2, said center point is coupled on one side to a reference potential (ground) via a series circuit comprising a capacitor C2 and a diode Dl (a diode or a zener) , and on the second side to the lamp (lamps) via a capacitor C3 and an inductance L2, moreover, said center point M is also connected, via a resistor R2, to an HB control unit for supplying pulse to the transformer Tl for driving the two switches Sl, S2. The HB control unit has a supply terminal VCC which is coupled to the connection point N between capacitor C2 and diode Dl, and detects the voltage changing at the point M by the resistor R2 to adjust the amplitude of pulse output. As demonstrated in figure 1, the output OUTl is used to drive the upside switch Sl, and output 0UT2 is used to drive the downside switch S2.

In this case, the time Toff between the moment when output 0UT2 is changing from high level to low level and the moment when the measured voltage at MS pin reaches a certain positive value rising from zero represents the real storage and fall time of the downside BJT S2, and said time Toff is used for base current regulation by keeping it at a constant value. Longer i

storage time of said BJT S2 caused by BJT tolerances or high temperature in normal operation is compensated by lower IC output voltage and vice versa. IC regulates both outputs each cycle step by step (one step per cycle) with the same level and there are certain steps between the minimum and maximum output voltage value. The amplitudes of the two outputs are the same and between turn-off of one output and turn-on of the other output there is a deadtime which is fixed to be a certain value .

Instead of the transformer also a semiconductor based half- bridge driver (discrete or integrated) can be considered. In principal, instead of BJT, any type of electronic switch could be used.

BJTs play the role of switch in the half-bridge circuit. In conduction they should be saturate to minimize the power loss therein, and not active inversely. Usually deeper saturation will result in longer storage time and vice versa. The MS pin signal is just used to measure the real storage and fall time of the downside BJT S2 for evaluating saturation status. However, the gains and storage times of the two BJTs in half bridge are usually not the same because of BJT tolerances. The gain of the upside BJT might be less than that of the downside BJT, and the real storage time might be shorter than that of the downside BJT. So when driving current is appropriate for the downside BJT, it might be not enough for the upside BJT, which could be active slightly, resulting in higher power losses of BJT switches and lamp flickering sometimes. In this regard, it is advised in the prior art to adjust driving transformer and base unit to make sure that there is enough driving current for the upside BJT, whose gain and storage time is minimal in BJT specification while the gain and storage time of the downside BJT is maximal (worst BJT combination) . However this solution usually make IC output kept minimal, that means there is no regulation function.

In addition, when both BJTs in the half bridge work normally, they are operating in zero voltage switching condition (almost

free of power loss, see the left side of table 1) . However there is always certain power loss in these BJT switches, which will make BJT switches hot. Temperature has influence on the storage time of BJT, and usually higher temperature makes storage time longer. While storage time becomes longer, the deadtime of IC outputs is kept unchanged, thus might be not enough then. As a result, before the voltage between the collector and emitter of a BJT falling down zero volt, the BJT might have already been turned on (the switch has power loss, see the right side of table 1) . More power loss in BJT will cause longer storage relevantly. It is a kind of vicious circle, which could destroy both BJT at the end. In this regard, it is advised in the prior art to adjust driving transformer and base unit to make storage time shorter relatively, and to improve cooling condition to keep temperature of BJT as low as possible. However this solution can not absolutely ensure that there is enough deadtime when both BJT switches are hot, and the gain and storage time of BJT are not symmetrical because of BJT tolerance.

Contents of the Invention

Therefore, the technical problem to be solved by the present invention is to provide an improved method for detecting the real cutting off signal of the BJT in an electronic ballast and a corresponding electronic ballast. Based on the detection of such real cutting off signal of BJT, better drive control within the electronic ballast can be realized through further improvement .

In accordance with the method for detecting the real cutting off signal of the BJT in an electronic ballast as provided by the present invention, said electronic ballast comprises a half-bridge circuit composed of a first and a second BJT connected end to end in series, the bases of said first and second BJTs being respectively controlled through a first and a second base unit so as to turn on said first and second BJTs in turn; an HB control unit for regulating the control pulses, which are respectively supplied to said first and second base

units, according to the voltage output signal at the output terminal of said half-bridge circuit, wherein said voltage output signal is on the one hand provided to a lamp and on the other hand is grounded via a capacitor and a backward diode, and the connection point between said capacitor and said diode is coupled to the supply terminal of said HB control unit. According to the present invention, said HB control unit measures the real cutting off signal of said first and/or second BJT through measuring the sharp slope of voltage at said connection point. Due to the fact that said sharp slope of voltage is more accurately closer to the transient position of voltage at point M than the voltage slope directly obtained at the original detection point M, so the real cutting off signal of BJT can be measured more readily and accurately.

Preferably, the first resp. second storage time of said first resp. second BJT can be obtained by measuring respectively the time between the flank of the control pulse for cutting off each of said first and second BJTs and the corresponding sharp slope of voltage appearing accordingly at said connection point. In this way, the first resp. second storage time of the first resp. second BJT can be detected more easily and accurately.

Preferably, said HB control unit compares said first and second storage times, and the shorter storage time is used to adjust the amplitude of IC output for base current regulation. In this way, the BJT with shorter storage time is supplied with appropriate current, and kept saturated in conduction. The other BJT with longer storage time will be saturated too in good conduction situation with minimal power loss, when it is turned on. It is also advantageous in this solution that if said shorter storage time is less than a constant value set inside or outside the IC, IC will increase the amplitudes of output pulses, and vice versa.

Alternative or addition to the above-mentioned solution, said HB control unit compares said first and second storage times, and the longer storage time is used to adjust the deadtime of

IC outputs to make sure that there is always enough and appropriate deadtime. Preferably, the deadtime is calculated and adjusted according to the longer storage time per cycle, so said deadtime can be calculated and adjusted according to, for example, the formula of deadtime = a x the longer storage time + b, wherein a and b are constant values. Alternatively, the deadtime is first kept constant, and it is increased to d when the longer storage time exceeds a constant value c, while if the longer storage time is less than the constant c, the deadtime is decreased to the original value.

The electronic ballast provided by the present invention comprises a half-bridge circuit composed of a first and a second BJT connected end to end in series, the bases of said first and second BJTs being respectively controlled through a first and a second base unit so as to turn on said first and second BJTs in turn; an HB control unit for regulating the control pulses, which are respectively supplied to said first and second base units, according to the voltage output signal at the output terminal of said half-bridge circuit, wherein said voltage output signal is on the one hand provided to a lamp and on the other hand is grounded via a capacitor and a backward diode, and the connection point between said capacitor and said diode is coupled to the supply terminal of said HB control unit. According to the present invention, said connection point is also coupled to the measurement pin of said HB control unit, such that said HB control unit can measure the real cutting off signal of said first and/or second BJT through measuring the sharp slope of voltage at said connection point. As mentioned in the above solution in respect of method, said sharp slope of voltage is more accurately closer to the transient position of voltage at point M than the voltage slope directly obtained at the original detection point M, so that the real cutting off signal of BJT can be measured more readily and accurately. Similarly, the advantageous effects as demonstrated for the refinements of the above-mentioned method are obviously applicable to the refinements of the electronic ballast of the present invention which will be described below.

Preferably, the HB control unit obtains the first resp. second storage time of said first resp. second BJT by measuring respectively the time between the flank of the control pulse for cutting off each of said first and second BJTs and the corresponding sharp slope of voltage appearing accordingly at said connection point. In this way, the first resp. second storage time of the first resp. second BJT can be detected more easily and accurately.

In said electronic ballast, said HB control unit compares said first and second storage times, and the shorter storage time is used to adjust the amplitude of IC output for base current regulation. If said shorter storage time is less than a constant value set inside or outside the IC, IC will increase the amplitudes of output pulses, and vice versa.

In said electronic ballast, said HB control unit compares said first and second storage times, and the longer storage time is used to adjust the deadtime of IC outputs to make sure that there is always enough and appropriate deadtime. Said HB control unit can calculate and adjust the deadtime according to the longer storage time per cycle. For example, said HB control unit calculates and adjusts the deadtime according to the formula of deadtime = a x the longer storage time + b, wherein a and b are constant values. Alternatively, said HB control unit can first keep the deadtime constant, then increase said deadtime to d when the longer storage time exceeds a constant value c, and decrease said deadtime to the original value if the longer storage time is less than the constant c.

In said electronic ballast, said connection point can be connected to the measurement pin MS of said HB control unit via a resistor.

The control pulses of said HB control unit can be coupled to the first and second base units respectively through a driving transformer so as to provide control pulse for cutting off said first and second BJTs in turn. Alternatively, the control pulses of said HB control unit can be coupled to the first and

second base units respectively through a semiconductor-based half-bridge driver so as to provide control pulse for cutting off said first and second BJTs in turn.

Description of drawings

The present invention will be explained in detail with reference to the appended drawings, wherein

Fig. 1 shows a part of the circuit of the prior art electronic ballast;

Fig. 2 shows a part of the circuit of the electronic ballast of the present invention;

Fig. 3 shows the graphs of the voltages measured according to the present invention;

Fig. 4 shows the graphs when measuring the time between the negative slope of output pulse OUTl of the upside BJT and the negative sharp slope at the connection point N (i.e., the real cutting off time TsI of the upside BJT) according to the present invention; and

Fig. 5 shows the graphs when measuring the time between the negative slope of output pulse OUT2 of the downside BJT and the positive sharp slope at the connection point N (i.e., the real cutting off time Ts2 of the downside BJT) according to the present invention.

Table 1 shows the zero voltage switching and non-zero voltage switching which might appear in the prior art when a BJT experiences temperature variation.

Preferred Embodiments

Fig. 1 and Table 1 have been described in the Background Art and will not be elaborated any more.

Turn to the electronic ballast of the present invention as shown in Fig. 2. Said electronic ballast is to great extent similar to the prior art electronic ballast as shown in Fig. 1. But the difference is that the measurement pin MS is connected to the connection point N between the capacitor C2 and diode Dl through a resistor R3, and is used for measuring the voltage

signal at the connection point N, which signal is shown in Fig. 3 as Ch4. It can be seen from the graph of Fig. 3 that voltage V-N (the voltage Ch4 at the connection point N) changes immediately at the starting points of positive and negative slopes of voltage V-M (the voltage at the point M) , that is, voltage V-N has a sharp changing (sharp slope) from zero to positive and a sharp changing from positive to zero. The positive and negative slope of said voltage V-M (the voltage at the point M) are caused by cutting off of BJT collector current. This characteristic means that the sharp slope signals of voltage V-N can be used to detect real cutting off signals of BJT, and thus to calculate the real storage time of BJT by comparison with cutting off slope of the driving pulse in further .

As showed in Fig. 4, cursor cursl is at the negative slope of output pulse OUTl, and cursor curs2 is at the negative slope of voltage V-N (the negative slope of the upside BJT collector current too) . The time between cursl and curs2 thus is defined as the real storage time TsI of the upside BJT Sl, which is 976.0-0.0=976. Ons here. Similarly, as showed in Fig. 5, cursor cursl is at the negative slope of output pulse OUT2, and cursor curs2 is at the positive slope of voltage V-N (the negative slope of the downside BJT collector current ChI too) . The time between cursl and curs2 is thus defined as the real storage time Ts2 of the downside BJT S2, which is 1.016-0.0=1.016μs here. It can be seen that the real storage time of the downside BJT is greater than that of the upside BJT, and TS2- TSl=1.016μs-976.0ns=40.0ns.

It is clear that connection point N is connected to the measurement pin MS of the HB control unit, such that said HB control unit can accurately measure the real cutting off signal of said first and/or second BJT Sl and S2 by measuring the sharp slope of voltage at said connection point N.

Furthermore, said HB control unit can obtain the first resp. second storage time TsI, Ts2 of said first resp. second BJT Sl, S2 by measuring the time between the flank OUTl, OUT2 of the

control pulse for cutting off each of said first and second BJTs Sl, S2 on the one hand and the corresponding sharp slope of voltage appearing accordingly at said connection point N on the other hand.

In order to make both of the two BJTs Sl and S2 have appropriate driving current in spite of the parameter tolerance, said HB control unit compares said first and second storage times, and the shorter storage time is used to adjust the amplitude of IC output for base current regulation. In this way, the BJT with shorter storage time is supplied with appropriate current, and kept saturated in conduction. The other BJT with longer storage time will be saturated too in good conduction situation with minimal power loss, when it is turned on, thus avoiding higher power losses of BJT switches and lamp flickering. Specifically, said shorter storage time is less than a constant value set inside or outside the IC, the IC will increase correspondingly the amplitudes of output pulses, and vice versa.

In order to avoid insufficient deadtime caused by longer storage time and thus destroying both BJTs Sl and S2 at the end in spite of the temperature change, said HB control unit compares said first and second storage times, and the longer storage time is used to adjust the deadtime of IC outputs to make sure that there is always enough and appropriate deadtime. Specifically, said HB control unit calculates and adjusts the deadtime according to the longer storage time per cycle. In particular, said HB control unit can calculate and adjust the deadtime according to the formula of deadtime = a x the longer storage time + b, wherein a and b are constant values. Alternatively, said HB control unit can first keep the deadtime constant, then increase said deadtime to d when the longer storage time exceeds a constant value c, and decrease said deadtime to the original value if the longer storage time is less than the constant c.

For the electronic ballast of the present invention, the control pulses of said HB control unit can be coupled to said

first and second base units respectively through a driving transformer Tl so as to provide control pulse for cutting off the first and second BJTs Sl and S2 in turn. Besides, said driving transformer Tl may be replaced with a semiconductor- based half-bridge driver.

It can be derived from the above description that the present invention has offered an electronic ballast operating more safely and a method for operating the same.