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Resistorless Precharging of Boost PFCs by Phase Angle Control of Thyristors

Frank Schafmeister, Michael Hufnagel, Peter Ide Delta Energy Systems, 59494 Soest, Germany, [email protected]

Abstract

Most AC/DC Power Supply Units (PSUs) and automotive on-board chargers (OBCs) which operate directly from the AC mains employ a PFC boost converter as front-end stage. The PFC stage regulates the AC side power factor close to one and at the same time it also controls the DC-link voltage, i.e. the input voltage for subsequent converter stages, to an adjustable constant value. Most PFC stages operate in boost mode and therefore require a precharge circuit for limiting the initial current required to charge up the DC-link capacitor (from 0V to a value close to AC voltage amplitude) prior to the regular PFC boost operation. Conventionally current limiting during the precharge interval is facilitated by very bulky resistors which are bypassed by a kind of switch (relay or semiconductor based) when the regular boost operation starts. Both, switches and precharge resistors are extra components which are exclusively used for initial precharging only and do require a considerable volume and cost effort. This paper proposes a method to totally omit the precharge resistors and moreover simplify the switch implementation. With just replacing two rectifying diodes by thyristors and providing an appropriate phase angle control precharge functionality is reached. The proposed concept is finally verified by measurements on a 3.3kW prototype.

1. Introduction

Boost PFC converter are the front-end stages of many of today’s AC/DC Power Supply Units (PSUs) and automotive on-board chargers (OBCs) which operate directly from the AC mains in the power range from 300W to several 10th of kW. The PFC stage regulates on the one hand the AC side power factor close to one (= power factor correction), on the other hand it also controls the DC-link voltage (UC in the following), which is the input voltage for the subsequent converter stages, independently of the actual power flow to a constant value. Most of those PFC stages operate according to the boost principle, i.e. the DC-link voltage UC has always to be larger than any instantaneous value of the AC input voltage uAC, which can be assumed to be sinusoidal over time and is described by

, (1)

where ÛAC denotes the mains amplitude and the mains angular frequency. A sinusoidal voltage system according to (1) often is also described by its rms value UAC which is given by

. (2)

PCIM Europe 2014, 20 – 22 May 2014, Nuremberg, Germany

ISBN 978-3-8007-3603-4 841 © VDE VERLAG GMBH · Berlin · Offenbach

Fig.1a depicts a standard boost PFC converter with all relevant quantities being marked. As mentioned above, in normal boost operation the condition

(3)

has to be fulfilled in principle. But before the initial start up of the boost PFC the DC-link capacitance C is usually discharged, i.e. UC=0. This means C has to precharged before starting the regular boost operation. Considering t=0 as precharge starting point, the precharge process has to transfer uC from

to

when TEndPrech defines the point of time when precharge is finished.

As soon as a voltage uAC≠0 is applied to the PFC input terminals of Fig.11 a charge current iAC will flow and autonomously charge up C - finally up to the level of the voltage amplitude ÛAC. Since in the charge current path of the main PFC circuit there are just diode elements aligned in conduction direction, actually no current limiting will be provided.

Even if the bypass diode DBP (dotted line) is not placed, the comparably small boost inductance LB would not sufficiently limit the in-rushing charge current. Therefore, the PFC circuits in Fig.1 include a dedicated precharge circuit (dashed boxes) which basic function is to limit the resulting charge current iAC by a defined precharge resistance R, i.e.

. (4)

Since on the other hand after precharging the regular PFC operation starts which usually impresses a significant continuous AC current with the rms value IAC, this current would cause considerable losses

, (5)

which normally would destroy the precharge resistor R thermally. For this reason it is the conventional practise, to bypass the resistor R with a controlled switch as soon as precharging is

Fig.1: The conventional Boost PFC precharge circuit drawn in a basic form (dashed box). A resistor R is paralleled to a controllable switch S. This switch can be implemented as an electromechanical relay or by different types of switchable power semiconductors like e.g. IGBT, MOSFET, Thyristor. Different locations of the precharge circuit within the Boost PFC topology are possible, for instance at the AC side (a) or at the DC side (b). Moreover further different locations are possible. Before starting the regular boost operation the precharge of the dc-link capacity C has to be performed, i.e. UC has to be increased typically from UC=0V to UC=ÛAC=Max(uAC).



finished and normal PFC boost operation is about to start (i.e. for t > TEndPrech). This switch can be implemented as a conventional relay (cf. Fig.1) or by different types of switchable power semiconductors like e.g. IGBT, MOSFET or thyristor (Fig.2 and [1]).

One common point of all the known state of the art implementations of precharge concepts is the power supply of the PFC’s control unit which is typically (only) fed from the DC-link capacitor C (cf. Fig.2b). Since before precharging (t ≤ 0) the DC link voltage is zero (UC = 0), the control unit is not powered up and therefore cannot take any active action at this point of time, i.e. there is no way to

switch Th1, Th2. or TB then. Switching actions (on Th1, Th2, TB or bypass switch S) have to happen right

after the end of the precharge interval for t > TEndPrech.

2. Principle of Resistorless Precharging by Phase Angle Control of Thyristors

The proposed concept of ‘Resistorless Precharging’ [2] is presented in Fig.3 (dashed box). The idea is the usage of phase angle control of the thyristors. This means right at the beginning of the precharge interval (t ≈ 0) switching actions in the form of thyristor ignition pulses have to be

Fig.2: Known implementation [1] of the Boost PFC precharge circuit (dashed grey lines) employing thyristors (Th1, Th2) and additional diodes (D1, D2). Th1 and Th2 are replacing 2 rectifying diodes (DR1, DR2 cf. Fig.1a). The precharge resistor R is still needed to limit the in-rushing charge current. (a): General power circuit. (b): The power supplying arrangement for the required control unit of the power circuit is additionally shown. Conventionally this control supply is fed from the DC link capacitor C, i.e. the available input voltage is (only) UC.

Th1 Th2

CuAC

UC

UAuxControlPower SupplyControl

Volt.Input 2

Volt.Input 1

uAC

s1 s2

uC

s1 s2

TB

uAC,Rect

Fig.3: Proposed concept of ‘Resistorless Precharging’ (dashed box). Compared to Fig.2 the precharge resistor R and the diodes D1, D2 are not needed anymore. This simplifies the hardware design, especially since R conventionally has to have a bulky volume. An auxiliary control power supply input voltage UAux which is not derived from UC is required for this concept. Optionally, in addition to this auxiliary voltage UAux also UC - as given in the conventional arrangement - can be used as alternative input for the control power supply (cf. grey connection line). Only during the actual precharge process itself UAux has to be used. Optionally the 2 thyristors Th1, Th2 could also be placed at different positions within the rectifying circuit.



employed. Since just small and controllable segments of the (rectified) AC voltage are effectively passed through to the dc-link capacitor C (cf. to Fig.4) it is possible to achieve very small, i.e. very limited charge currents iAC. For this reason the precharge resistor R for the current limitation is not longer required in this concept. Actually the whole precharge path (dashed box in Fig.1) is not needed anymore. This indeed simplifies the hardware design, especially since R for thermal reasons conventionally has to have a bulky volume when the resulting power dissipation at R (into heat) is taken into account appropriately.

To be able to generate the thyristor ignition pulses right at the beginning of the precharge interval (i.e. when UC≈0) an auxiliary input voltage UAux is required to supply the control unit and the thyristor driving circuits with power. Accordingly UAux should be more or less constant and especially not derived from UC. In the application example of automotive OBCs we will inherently find this situation – the available low voltage (12V) battery supplies UAux in this case. It should be noted that the Control Power Supply in Fig.3 has of course to be capable of handling the voltage level UAux as input voltage. Fig.3 also introduces the thyristor control signals s1, s2. These signals whose waveforms are exemplary depicted in Fig.5b are generated by the control unit in dependency of the input quantity uAC and optionally also in dependency of UC.

In Fig.4 the basic principle of Phase Angle Control of thyristors as proposed solution for the task of precharging the dc-link capacity C (cf. Fig.3) is visualized over the first 4 mains half periods of the

precharge interval. There, the angle [0...180°] shall represent the relative phase angle within

each mains half period, whereas ( t) is the absolute phase angle. As it is generally known, a thyristor is a power electronic valve which only can be turned on. So, once it is ignited and moreover a positive voltage is applied across, it is in the conducting state and it will stay conductive until the next AC voltage zero crossing will occur.

Fig.4: The principle of Phase Angle Control of thyristors as method to achieve a Resistorless Precharging of the

dc-link capacity C: When starting the precharge process (I) the phase angle of thyristor ignition ( , cf. Fig.5c) has to be just marginally smaller than 180°, i.e. thyristor ignition has to be initiated shortly before the zero-crossing of the mains voltage uAC. In the up following mains half period (II) the thyristor ignition has to happen

marginally earlier than in the previous half period. So the ignition phase angle has to slightly decrease from one mains half period to the next one. As a consequence the maxima of uAC,Rect continuously increase.



When starting the precharge process in the first mains half period (I) the phase angle of thyristor

ignition ( , cf. Fig.4c) has to be just marginally smaller than 180°, i.e. thyristor ignition has to be initiated shortly before the zero-crossing of the mains voltage uAC. In the up following mains half period (II) the thyristor ignition has to happen marginally earlier than in the previous half period (I).

So the ignition phase angle has to slightly decrease from one mains half period to the next one. As a consequence the maxima of uAC,Rect continuously increase. Since there is no significant voltage drop between uAC,Rect and the unloaded dc-link voltage uC, latter increases stepwise with the aforementioned voltage maxima, i.e. uC= Max(uAC,Rect) as it is visualized by the grey waveform in Fig.3. The resulting AC current peaks (not shown) which occur with each voltage step of uC are

limited and can indeed be controlled by the amount of decrease of ignition angle between the

single mains half periods ( n n-1). The smaller the absolute value of decrease | the less are the resulting AC current peaks during precharging.

Fig.5: ‘Resistorless Precharging’. A whole precharge cycle with corresponding thyristor control signals s1, s2 is

illustrated. (a): When uC=ÛAC=Max(uAC) has reached at the thyristor ignition angle =90°, the dc-link voltage uC

does not increase anymore, even if would be decreased further. (b): The pulse width of the control signals continuously increases from 0 to 90°. s1 (solid black line) is activated in the mains half periods when uAC>0 is given, whereas s2 (dashed grey line) is released only during the negative half periods uAC<0. (c) gives an example of how to generate the control signals s1, s2. When the relative phase angle is larger than the ignition

angle ( > ) the corresponding control signal has to be set high. It should be mentioned that the ignition angle

step size ( between 2 neighbouring mains half periods is chosen exemplary here in order to support an appropriate visualization. Practically it should be far less. Also, this design parameter directly defines the duration (and/or number of mains half periods) until the precharge process is finished.



The waveforms of a whole precharge interval are illustrated in Fig.5 which also includes the

corresponding control signals s1, s2 being marked in the circuit diagram of Fig.3. As Fig.5a shows, the ignition angle =90° leads to the maximum dc-link voltage level available during precharging, i.e.

Max(uAC,Rect)=ÛAC=uC. The further decrease of lets uC stay constant at ÛAC, which is also directly

obvious from Fig.6a. Moreover these facts lead to the conclusion that the angle step size is a design parameter which defines the duration TEndPrech of the whole precharge interval. The thyristor control signals with from 0° to 90° continuously increasing pulse width are depicted in Fig.5b.

Thereby, directly at the start of the precharge interval the ignition angle has to be just marginally smaller than 180°, which is also noticeable from Fig.5c. In order to reduce leakage losses that thyristor which is exposed to a negative voltage lying across its anode-cathode terminals should preferably not receive any ignition pulses. So, s1 shows pulses in the positive mains half periods only (uAC>0), whereas s2 does it exclusively during the negative half periods (uAC<0).

Fig.6: Two variants of start condition for the normal boost operation which follows after the ‘Resistorless

Precharging by Phase Angle Control of Thyristors’. (a): The thyristor ignition angle is continuously decreased

also after uC=ÛAC at =90°. The normal boost operation in general should only be performed when =0 is given and the thyristors therefore act as conventional rectifying diodes would do (i.e. conducting over the whole mains half period). (b): To accelerate the precharge process (by a factor of 2), the fulfilled condition uC=ÛAC can

be used to directly set =0 in the next mains half period and to start the boost operation immediately then.



In Fig.5c an example of a method to generate the control signals s1, s2 is presented. When the

relative phase angle is larger than the ignition angle ( > ) the corresponding control signal has to be set high when at the same time the above mentioned voltage condition (uAC>0 for s1, uAC<0 for s2) is fulfilled.

Two variants of precharge end condition - which at the same time is the boost operation start condition – are documented in Fig.6. In general the normal PFC boost operation should only be

performed when (at least) =0 is given and the thyristors behave as conventional rectifying diodes would do, which means they conduct over the whole mains half period. If the ignition angle would

stay constantly at a value larger than zero ( const. the drawn AC current iAC in PFC operation never could be real sinusoidal and as a consequence the Total Harmonic Distortion (THD) and Power Factor (PF) values could not reach satisfying levels. Practically, even slightly negative ignition angle

values [-10°...0] are applied in steady state PFC operation to avoid any delay in the ignition pulse and to guarantee the thyristor gets conductive as soon as a positive voltage lies across its anode-

cathode terminals (at =0). For the sake of simplicity in the following text and figures it will be stated

that =0 is given during the regular PFC boost operation.

The first variant is described by Fig.6a where the thyristor ignition angle is continuously decreased

- also after uC=ÛAC is reached at =90°. When the precharge end condition =0 is fulfilled, the start of the regular boost operation is initiated. This method requires as input quantity just uAC in order to

determine the angles and .

The second variant being presented in Fig.6b does not wait until =0 is reached by continuous

equidistant decrementing steps . The whole precharge interval can be accelerated by a factor of

2, when the condition uC=ÛAC (and/or =90°) is constantly checked and when being true, =0 is directly set for the next mains half period. With the beginning of this next half period the regular boost operation is started. This method uses as input quantities uAC and preferably also uC.

3. Extension of the Principle to Bridgeless Boost PFC Topologies

The proposed principle of ‘Resistorless Precharging by Phase Angle Control of Thyristors’ [2] can be also directly applied to all types of Bridgeless Boost PFCs including e.g. H-PFC [3] or Totem-Pole PFC.

SB

SiC

=SiC-JFET

OR OR OR

CuAC UC

ControlPower SupplyControl

SB1

SB2

Volt.Input 2

Th1

Th2

UAux

Volt.Input 1

CuAC UC

Volt.Input

SB1

SB2

R

S

DR1

DR2

ControlPower SupplyControl

a) b)

c)

Fig.7: Bridgeless Totem-Pole PFC with the conventional (a) and the proposed (b) precharge circuit (each in the dashed box). The proposed concept (b) does not require any precharge resistance R. For this PFC topology the proposed precharge circuit can be implemented by simply replacing the 2 conventional rectifier diodes (DR1, DR2) by the thyristors Th1, Th2 (aligned in same polarity), which means minimum effort. The control of these thyristors is identical to the principle being described in section 2. The depicted boost switches sB1, sB2 shall represent various power semiconductor technologies (c).



In the following all these Bridgeless PFC topologies shall be exemplary represented by the Totem-Pole PFC in Fig.7a which can be implemented by various technology types of boost switches sB1, sB2 as given in Fig.7c. The 2 conventional rectifier diodes DR1, DR2 can simply be replaced by 2 thyristors which directly yields the proposed implementation of the Resistorless Precharge circuit (dashed box) in Fig.7b.

4. Experimental Results on HEV On Board Charger and Outlook

Fig.8 shows experimental results from a 3.3kW prototype OBC verifying the proposed approach of

Resistorless Precharging. The first variant is used with different decrementing step widths

Obviously when comparing Fig.8a vs. Fig.8b, the maximum peaks of precharge current decline

(almost linearly) with the smaller ignition angle step width Moreover, the precharge current

spikes continuously reduce in magnitude with decreasing until =90° is reached. This behaviour is plausible since also the causing DC-link voltage steps (cf. uC in Fig.6) reduce in the same way.

Considering this, a further future control option worth to mention here is to dynamically increase

the decrementing steps from one mains half period to the next one instead of keeping always constant as given in Fig.8. This would target at keeping the precharge current spikes at a more constant, high level and thus leading to a minimum precharge duration.

5. References

[1]: Kolar, J.W., Korb, W., European Patent EP1186093B1 “Device and Method for Line-Voltage Dependent Thyristor Controlled Pre-Charging of Output Capacitors in a Three-Level Pulse Rectifier System”, filed: May 17, 2000.

[2]: Hufnagel, M., Schafmeister, F., European Patent Application EP13192249.4 “Resistorless Precharging of PFC Boost Rectifier by Phase Angle Control of Thyristors”, filed: Nov. 8, 2013.

[3]: Liu, T., Xin, X., Zeng, J., Ying, J., Zhao, W., US Patent US7605570B2 “Bridgeless PFC Converter with Low Common-Mode Noise and High Power Density”, filed: Apr. 4, 2007.

iACuC

(10A/Div)(100V/Div)

(a)

BoostPrecharge

iAC

(10A/Div)(b)

Precharge

AC Connecting

Fig.8: Measured AC current iAC of 3.3kW prototype OBC at 230VAC during the precharge process.

(a) First implementation with 55 steps per 90°, i.e. ≈1.6° showing iAC and uC over 2s. Current peaks of ≈30A result.

(b) Second implementation with 110 steps per 90°, i.e. ≈0.8° showing iAC over 4s. Current peaks of ≈15A result.



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