Motor de Búsqueda de Datasheet de Componentes Electrónicos |
|
ML4802 Datasheet(PDF) 7 Page - Micro Linear Corporation |
|
ML4802 Datasheet(HTML) 7 Page - Micro Linear Corporation |
7 / 15 page ML4802 7 August 2000 Datasheet On an instantaneous basis, an increase in VOUT above its programmed value will cause the error voltage presented to VDC to decrease. This will shut off PWMOUT to keep the loop in regulation. If the output voltage goes below its intended level, VDC will increase. When the feedback voltage VDC rises above VTH2, PWMOUT is re-enabled causing the output voltage to increase. This series of actions will repeat, maintaining the average VOUT at its design value. Since the PWM error amplifier gain is quite high in the average configuration, this action introduces no appreciable ripple on the PWM’s DC output(s). One item to note here is that, to keep the pulse skipping action as clean as possible (that is, to prevent pulse grouping), a relatively fast error amplifier with an electrically quiet feedback path to VDC is desirable. When the PWM enters its pulse-skipping mode, the PFC is shut off completely. The PWM then runs off of the energy stored in the PFC buss capacitor. During this period, the voltage on the buss capacitor will decay. When VBUSS falls below a user-set threshold VPFC1 (typically 382V), the PFC is turned on again, charging its output capacitor back to a higher voltage VPFC2 (typically 425V). Simultaneously, the threshold to which VDC is compared is switched back to VTH1. As soon as the output voltage of the PFC exceeds VPFC2, the PFC shuts off and VDC is again compared to VTH2. This cycle repeats as long as the power consumption from the PWM remains below the Green Mode threshold. Exiting Green Mode The ML4802 enters Green Mode at any time that VDC < VTH1. In order to reliably exit Green Mode, VTH1 must be used as the exit criterion as well (using VTH2 as a comparison voltage to exit Green Mode would eliminate the part’s ability to skip pulses throughout the Green Mode power range). Therefore, once the voltage on VDC has set the part into Green Mode operation, the ML4802 can only exit Green Mode when the PFC is recharging the buss capacitor. As noted above, VDC is compared against VTH1 during the PFC recharge time. Another way of viewing this is as follows: every time the PFC turns on, the ML4802 exits Green Mode, and will either return to Green Mode or remain in continuous-mode operation depending upon whether the voltage on VDC exceeds VTH1. Note that this means that there will be brief periods of continuous PWM operation even while the output power drawn from the PWM is within the Green Mode range. This is a normal and harmless consequence of the ML4802’s Green Mode logic. GREEN MODE THRESHOLD To a first approximation, the Green Mode Threshold as a percentage of the PWM’s maximum rated power output is given by: PGMT = (VGMT/VCURRENT LIMIT(PWM)) x POUT(MAX) PGMT @ (0.25V/1.5V) = 0.167 x POUT(MAX) For example, a flyback supply designed for 100W maximum output will nominally enter and exit Green Mode operation at 17W. Similarly, a 200W forward converter would have a Green Mode threshold of about 34W. In actual designs, the Green Mode threshold will often be at a slightly lower power level than is given by this simplified equation. This is principally due to the fact that VFB is an average-responding voltage, while POUT is inferred from the instantaneous peak current through RSENSE(PWM). On a short-term basis, the output current demand as sensed by VFB is essentially a DC level. This is not true of V(RAMP1), however: V(RAMP2) is given by (RSENSE(PWM) x IPRIMARY(PWM)), which for most designs is a combination of DC (pedestal) and peak (ramp) currents. It is the ramp current portion of IPRIMARY(PWM) which causes real-world designs to typically enter Green Mode at several percentage points lower output power than would otherwise occur. POWER FACTOR CORRECTION Power factor correction makes a non-linear load look like a resistive load to the AC line. For a resistor, the current drawn from the line is in phase with, and proportional to, the line voltage, so the power factor is unity (one). A common class of non-linear load is the input of a most power supplies, which use a bridge rectifier and capacitive input filter fed from the line. The peak- charging effect which occurs on the input filter capacitor in such a supply causes brief high-amplitude pulses of current to flow from the power line, rather than a sinusoidal current in phase with the line voltage. Such a supply presents a power factor to the line of less than one (another way to state this is that it causes significant current harmonics to appear at its input). If the input current drawn by such a supply (or any other non-linear load) can be made to follow the input voltage in instantaneous amplitude, it will appear resistive to the AC line and a unity power factor will be achieved. To hold the input current draw of a device drawing power from the AC line in phase with, and proportional to, the input voltage, a way must be found to prevent that device from loading the line except in proportion to the instantaneous line voltage. The PFC section of the ML4802 uses a boost-mode DC-DC converter to accomplish this. The input to the converter is the full wave rectified AC line voltage. No filtering is applied following the bridge rectifier, so the input voltage to the boost converter ranges, at twice line frequency, from zero volts to the peak value of the AC input and back to zero. By forcing the boost converter to meet two simultaneous conditions, it is possible to ensure that the current which the converter draws from the power line matches the instantaneous line voltage. One of these conditions is that the output voltage of the boost converter must be set higher than the peak value of the line voltage. For the ML4802, a good value to use is 425V DC out, to allow for a high line of 270V AC while in Green Mode. The other condition is that the current which the converter is allowed to draw from the line at any given instant must be proportional to the line voltage. The first of these requirements is satisfied by establishing a suitable voltage FUNCTIONAL DESCRIPTION (Continued) |
Número de pieza similar - ML4802 |
|
Descripción similar - ML4802 |
|
|
Enlace URL |
Política de Privacidad |
ALLDATASHEET.ES |
¿ALLDATASHEET es útil para Ud.? [ DONATE ] |
Todo acerca de Alldatasheet | Publicidad | Contáctenos | Política de Privacidad | Intercambio de Enlaces | Lista de Fabricantes All Rights Reserved©Alldatasheet.com |
Russian : Alldatasheetru.com | Korean : Alldatasheet.co.kr | Spanish : Alldatasheet.es | French : Alldatasheet.fr | Italian : Alldatasheetit.com Portuguese : Alldatasheetpt.com | Polish : Alldatasheet.pl | Vietnamese : Alldatasheet.vn Indian : Alldatasheet.in | Mexican : Alldatasheet.com.mx | British : Alldatasheet.co.uk | New Zealand : Alldatasheet.co.nz |
Family Site : ic2ic.com |
icmetro.com |